Implementing code review feedback.

Merge pull request #575 from bsermons/develop ...

Fix error when copying sheets with certain image anchors

Move test to XLWorksheetTests.cs

Fix error when copying sheets with certain image anchors

Add unit test for at least one previously mistreated formula

Invalid Print Area fix (issue 578) ...

The modified code will not try to add an (invalid) print area to an unnamed worksheet, avoiding the unhandled exception.

separate test cases for FLOOR.MATH ...

this would allow easier detection of what's failing and follows the principle of one assertion per test.

improve FACTDOUBLE and add unit tests ...

FACTDOUBLE now throws correct exceptions and

improve Fact (throw correct exceptions on invalid input), add unit tests

add unit test for EXP function

unit tests for EVEN

unit tests for DEGREES function

add unit testeds for COSH

add unit tests for COS

improve COMBIN and add unit tests ...

COMBIN did not handle non-numeric values correctly. Should throw NumberException (#NUM in Excel). Returned 0 for any non-numeric parameter instead before

implement BASE and corresponding unit tests

throw correct exceptions on ATANH and add unit tests

fix ATAN2 implementation and add tests ...

Math.Atan2() expects y as first parameter and x as second, while in Excel x is first,
throw "correct" exception on input (0, 0)

add unit tests

add unit tests for ATAN function

add unit tests for ASINH

improve ASIN: throw NumberException for values not in range for Excel ...

...add unit tests for ASIN

implement ARABIC and unit tests

restrict valid input range to [1..) as in Excel, add unit tests.

fix ACOS implementation: in Excel only defined in range [-1, 1] ...

if input is not in range, throw exception.

adding unit tests for ACOS

ABS was implemented already, adding unit tests.

As AppVeyor complains about C#7 features, let's tell VS to use C#6.

Merge pull request #567 from jongleur1983/feature/moreTrigonometry ...

Implement more (all remaining?) trigonometric functions

implement SECH function and corresponding tests

implement SEC and corresponding unit tests

implement ACOTH function and corresponding unit tests