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.

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

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

* implement SumIfs and some tests

* implement CountIfs and some unit tests

apply code formatting and escape & in XMLDoc comments correctly.

#548 tests for reference example from Excel documentation, failing yet ...

,,,as they use patterns not yet covered by CalcEngine,

Merge branch 'develop' into feature/combina

fix Tests for COMBINA function

Correct INT function ...

The current behavior was not the same as the one described on the office website https://support.office.com/en-us/article/INT-function-a6c4af9e-356d-4369-ab6a-cb1fd9d343ef

Merge remote-tracking branch 'origin/develop' into feature/cot

implement COMBINA function and corresponding tests.

Merge branch 'feature/cot' into develop

implement COT function (co-tangens) and corresponding tests ...

special cases are multiples of PI

Merge branch 'develop' into feature/cosh

remove invalid test input (radix 1)

changes according to PR review remarks: ...

- change tests
- throw Exception instead of returning error code
- fix double comparison

#52: implement DECIMAL function