Upgrade project files to SDK-style format compatible with dotnet

[adecler]

Issues addressed by this PR

Fixes #128

• 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 upgraded the framework to netstandard2.0 as I felt 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

[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

[adecler]

@BHoMBot check installer

[bhombot-ci]

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

• check installer

[michaelhoehn]

Functions as expected, happy to merge.

[michaelhoehn]

@BHoMBot check ready-to-merge

[bhombot-ci]

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

• check ready-to-merge