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

implement ACOT function and unit tests

implement CSC function and tests

implement Coth and tests

Breaking change: Change XLCellValues to XLDataType

Merge pull request #564 from igitur/issue536-footer-causes-exception ...

Fix header/footer parsing.

Fix header/footer parsing. Fixes #536

Feature/sumifs and countifs (#561) ...

* implement SumIfs and some tests

* implement CountIfs and some unit tests

Save calculated column formula in table field (#528) ...

* Save calculated column formula in table field

* Add data format id to table columns if data format is consistent.

* Major improvement to the handling of styles. Drops IXLFill.PatternBackgroundColor!

* Remove property changed vars on XLFill

Merge pull request #555 from jongleur1983/548-conditionsForStrings2 ...

#548 enhance string condition capabilities of CalcEngine