Fix issues with copying worksheets across workbooks (#839) ...

* Tests for copying worksheets added

* Add tests for pictures and data validation rules

* Fix test for styles copying

* Extract copying logic into dedicated classes (XLPicture, XLTable, XLNamedRange). Implement copying of conditional formats.

* Implement failing scenario for existing test

* Fixed copying conditional formats

* Fixed copying of merged ranges

* Tests simplified as saving issues must be addressed separately

* Fixed saving the row heights on copied worksheets

* Add tests for copying tables and selected ranges. Fix copying selected ranges.

* Fix copying page setup

* Removed debugging code

* CopyTo and WithoutWorksheet method removed from public API. A new constructor for XLConditionalFormat added. Formatting issues fixed.

* Copying table with or without data implemented. Tests added.

* Revert removing public methods

* Copying pictures within a worksheet implemented. Tests added.

* Tests for named ranges copying added.

* Implement XLPicture.Duplicate method.

* Update doctext

* Fix IXLPicture.CopyTo signature

Multi-targeting: net40, net461, netstandard2.0 (#315) ...

* Convert to Visual Studio 2017 format.

* Force VS2017 appveyor image

* Add MyGet source before CI build

* Update artifacts

* Implement .NET standard as far as possible.
System.Drawing.Bitmap is still missing - hence the XLPicture API is incomplete

* Change to VS 2017 Preview image to support .NET Standard 2

* Add APPVEYOR environment variable support

* Improve color support and more

* Add a basic test for OpenXml - which currently breaks in .NET Standard 2.0

* Use OpenXML 2.7.2 for all targets

* Replace net452 with net40 build

* Try netstandard where possible

* Update NuGet on appveyor before building

* Implement MetadataExtractor to get image dimensions and format

* When setting culture, do it for every time a test is run.

* Fix dependency issue

* fix support for TIFF files

* Default the image position to top left if position hasn't been set.

* Test all targets

* Update test framework

* Add .editorconfig files back in projects

* Disable OpenXML test on AppVeyor until bug is fixed

* Better approximation of font sizes on netstandard2

* Workaround for OpenXML bug as per https://github.com/OfficeDev/Open-XML-SDK/issues/235

* Fix for DeflateStream not being seekable. Copy to MemoryStream

* Exclude Excel temporary files when building

* Use CoreCompat for measuring fonts on netstandard - remove lots of conditional code

* Reindent file

* Upgrade NUnit

* Add support for .NET 3.5

* Fix for NET35

* Remove explicit test targets. Should work on appveyor now.

* Add Microsoft.CSharp where necessary.
Remove net40 from tests, for now.

* Skip OpenXML document properties test

* Use System.Drawing.Common instead of MetadataExtractor and CoreCompat

* Sign projects

* Specify test assemblies in appveyor.yml

* Comment out platform in order to get CI to correctly detect test assembly path. as per https://github.com/appveyor/ci/issues/1746\#issuecomment-339187046

* Remove .NET3.5 build from artifacts

* Add net40 to tests

* Add reference to Microsoft.CSharp for net40

* Switch net461 to net46
Remove NU1605 warnings
Clean up dependencies

* Generate documentation file

* Ignore warning CS1591: Missing XML comment for publicly visible type or member

* Reenable some System.Drawing features for netstandard2

* Upgrade FastMember.Signed to v1.3.0

* Upgrade NUnit to 3.10

* Upgrade test framework

* Remove framework distinction for netcore

* Collapse empty XML elements for comparison - netcore and netframework versions of OpenXML save the internals differently.

* Copy text.xlsx to output directory

* Enable test for netstandard too.

* Refactor TestHelper.StripColumnWidths

* Align conditional compiler constants for netcore/netstandard to _NETSTANDARD2_0_

* Change documentation file declaration to enable build on Linux

FIX Pictures not being deleted (#674) ...

* Failing test for deleting pictures

* Process pictures deleting from a file

* Deleted pictures removed from the interface

Set picture Id correctly

Allow duplicate picture names when loading a file, but don't allow duplicate Ids

Can now delete pictures

Move test to PictureTests

More improvements

Unknown changes - something to do with line endings?

More image API improvements.

First rework of Picture API

Add tests for XLPicture and XLMarker classes ...

Tests mostly methods that aren't just direct setters and getters using the
known values of the resource image.