Issue 46: Switch netcoreapp2 to netstandard2 for libraries. …

[RyanGrange]

Description

Update for issue 46:
Switch netcoreapp2 to netstandard2 for libraries.
Removed unused using that would have required netcoreapp3.1 to compile.
Fixed string to char because netstd2 didn't allow for strings as char arrays.
Added .NETStandard2.0 dependency to nuspec.

Checklist

• An issue was first created before opening this pull request
• The new code follows the contribution guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests to ensure that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[aalhour]

Hi @RyanGrange, which operating systems did you test this change on?

[PatrykOlejniczak]

I apologize for the late check.

NetStandard is best option for us, because we shouldn't use any specific API.

I checked that on Windows 10, LGTM