Upgrade project files to SDK-style format compatible with dotnet

[adecler]

Issues addressed by this PR

Fixes #584

• The project files were still using the legacy .NET Framework project files and needed to be upgraded to SDK-style format compatible with dotnet.
• I have not upgraded the framework to netstandard2.0 as I was not confident this would be compatible for this repo and its dependencies.

Tests

• Make sure the solution build with dotnet build
• Run your end of milestone testing procedure

[adecler]

@BHoMBot check compliance

[bhombot-ci]

@adecler to confirm, the following actions are now queued:

• check code-compliance
• check documentation-compliance
• check project-compliance
• check branch-compliance
• check dataset-compliance
• check copyright-compliance

There are 25 requests in the queue ahead of you.