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

Merge pull request #572 from jongleur1983/feature/functionsAndTests-MathTrigStartingWithAtoB-exceptAggregate ...

Add all missing functions starting with A and B, except AGGREGATE

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

Merge pull request #570 from b0bi79/outofmemory ...

OutOfMemoryException. Some range disposing.

OutOfMemoryException. Some range disposing.

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.

Merge remote-tracking branch 'ClosedXML/develop' into develop

Merge pull request #568 from jongleur1983/feature/RestrictCSharpVersion ...

Feature/restrict c sharp version

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

Merge remote-tracking branch 'ClosedXML/develop' into develop

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

Implement more (all remaining?) trigonometric functions

fix C# version compatibility issue - thanks for the hint ;)

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