Update APRS parser from .NET Framework 4.5 to .NET Standard

[CBielstein]

The APRS parsing code was originally implemented for .NET Framework 4.5 as a portable class library. We should move this in to the future with .NET Standard, which is the much more modern (and much easier) model for cross platform libraries.

Honestly, the fastest way to do this is probably to just delete the .csproj file and create a new one with dotnet new classlib from within the directory. Likely want 2.1 instead of 2.0, so change that if appropriate inside the .csproj after creating. For the unit tests, do a similar process but dotnet new xunit

For this issue to be done:

• [ ] APRS parser project should target .NET Standard 2.1
• [ ] APRS parser unit test project should target netcoreapp3.1 (this is the .NET Core 3.1 runtime target)
• [ ] Building each project should succeed (dotnet build in each directory modified here)
• [ ] Running tests should succeed (dotnet test in the directory with the unit test project)

[CBielstein]

In the test directory, we want to create a new xunit test project with dotnet new xunit

[CBielstein]

Geocoordinate is referenced in the code and will fail compilation because it doesn't exist there. To get around this, add the following nuget package to the project:

Package: https://www.nuget.org/packages/GeoCoordinate.NetCore Install command: dotnet add package GeoCoordinate.NetCore --version 1.0.0.1 (run from the project directory in src)

[CBielstein]

For the unit test project, like we discussed you should:

1. Make a new xunit project instead of classlib dotnet new xunit (after deleting the old Properties directory and old .csproj)
2. Add reference to the APaRSer project itself using dotnet add <project> reference <targetproject>

[eddywine]

@CBielstein I have now pushed these changes