Fix/clean up net projects

[Apollo3zehn]

changes:

• replace .NET Standard, .NET, .NET Core and PCL projects with: .NET Standard 1.0, .NET Standard 1.3 and .NET 2.0 combined in a single project via <TargetFrameworks>netstandard1.0;netstandard1.3;net20</TargetFrameworks>
• Adapt the #if NETSTANDARD directives
• Adapt the nuspec file to reflect the new list of projects
• Remove https://github.com/mariuszgromada/MathParser.org-mXparser/blob/43d2b339f6c41be56a0ceb6ed885b6739cac0821/CURRENT/c-sharp/exe-lib-tests/Run-Tests-Reg/RunTests.cs#L13-L15 because the PCL project does not exist anymore
• Update Test projects to NET 4.7.2 since that is the first version that fully supports .NET Standard 2.0 (not so important change :-) )
• Remove obsolete test projects

The following table gives a rough overview about the framework dependencies and why (in my opinion) only three target frameworks are required:

Framework	Compatible to
net20 to net40	net20
net45 to net472	netstandard10
net46 to net48	netstandard13
netcoreapp10 to netcoreapp11	netstandard10
netcoreapp20 to netcoreapp30	netstandard13
PLC	netstandard10 / netstandard13

@mariuszgromada:

• What is the purpose of the mXparser-00-Current project? Is it for testing?
• I am not sure how you generate the nuget package. I was able to compile the project and then tried to generate a package via dotnet pack but I got the error that I have no .NET 2.0 installed, though I have it. I hope you can generate the package successfully (since compilation is successful).

[mariuszgromada]

Thank you!! I am going to verify and merge :-)

mXparser-00 is a project where i make development and all the testing (changing properties etc.)

Nuget package is generated by nuget explorer - a kind of visual tool