Support for .NET Standard 2.0 / .NET 4.6.1

[bobbymcr]

Add support for .NET Standard 2.0 (in all the core libraries) and .NET 4.6.1 (in the OMS and test projects).

Summary of changes:

• Update all projects to SDK style; set GenerateAssemblyInfo to false to avoid having to touch existing AssemblyInfo files
• Use PackageReference instead of packages.config
• Update .sln file to remove unnecessary build targets (assume AnyCPU)
• Upgrade to newest NUnit
• Modify path prefix in OrcaMDF.RawCore.Tests.BaseFixture
• Move TestHelper out of core framework and include it as a linked file in all the test projects (this is needed because the code used here is not compatible with .NET Standard)