Refactoring XSSFRow

[artem-iron]

Code clean-up, refactor into more of C# style code, use expression body style for properties and methods where applicable, use 'var' instead of explicit types where possible, add curly brackets for clarity in places where they were omitted, re-arrange members into regions.

Change javadoc to c# xml documentation.

No changes in code itself or logic, only refactoring.

[PBrunot]

I find your version much easier to read.
I had a doubt about the usage of => with older versions of the framework supported by NPOI ( .net framework / C# version 7.3 ).
This feature was added in C# 7.0 (source: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/expression-bodied-members)
So there is no language compatibility issue.
I suppose the appveyor should have caught it anyways, but better safe than sorry.

[artem-iron]

@PBrunot I'm a bit conflicted on the use of expression body myself, it makes code a little less cluttered, but does it help readability or hurts it? I'm not sure, to me it seems like neither. But since it does reduce clutter - decided to roll with it.

The language support, of course, is a legitimate issue, but I test the code on multiple targets including .net framework 4.6.2, and no language compatibility issues were raised while testing. Anyway, not married to the idea, let's see what @tonyqus (and/or others) think and if need be, I'll switch it back to full method bodies.

[tonyqus]

To be honest, I'm not a fans of expression body and var. NPOI used to support very old .NET framework (such as 2.0 and 3.x). I have a habit to avoid using new C# syntax feature since it may break the compilation.

I'm fine with the following changes:

• Convert javadoc documentation to csharp xml documentation in XSSFRow
• Re-arrange the members of XSSFRow.

[artem-iron]

@tonyqus I've addressed your comments in the recent commit. Thanks for the input!

[tonyqus]

LGTM