Improve fills comparison

Possible proof of concept for handling transparent fills (by @igitur).

Compare worksheet style with the Normal style rather than Default on saving.

Apply style Normal to a worksheet by default.

Failing test added

Optimize cells loading (remove conversion row number and column number to a string address that has to be parsed afterward)

Remove redundant uppercasing column letter

Remove redundant style conversion

Improve workbook load performance by eliminating redundant cell style instantiation

Add tests for missing and not supported file extension. Fix check for missing extension

Disallow negative row or column count to be inserted (#857) ...

* Disallow negative row or column count to be inserted

* Disallow zero columns or rows to be inserted as well

* Tests for zero numbers added. Typo fixed.

Remove legacy part to download new NuGet - kept auto-updating part.

Correctly clear OpenXML cell value if ClosedXML cell value is empty

Get cell by full address (#825) ...

* Failing test added

* Obtaining cell by a full address fixed.

* Tests for non-existing addresses added

* Add more exotic test cases

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

Build Nuget package on AppVeyor

Remove unreachable code; disable warning for filling obsolete property

Treat warnings as errors.

Support table sources for pivot tables

Update some tests with new methods

Remove CreatePivotTable overload that doesn't provide a name

Fix XLRangeBase.CreatePivotTable destination sheet - must equal targetCell's

Simplify adding table to pivot table

Rename IXLPivotTables.AddNew to Add and make the old methods obsolete

Remove IXLPivotTables.Add() - wasn't used

Add method AddNew to add pivot table directly from table

Remove ArgumentException from XLCell.Value ...

Add possible ArgumentException when changing XLCell.DataType

Simplify XLCell.cs()

Load file's cached value into XLCell.CachedValue - need to some refactoring of the internal parsing in XLCell

Use autoproperty for XLCell.Worksheet