Extensive updates to support multiple DSS instances, GR API, and exception mapping.

[PMeira]

• Use the new DSS Context API, available in the upcoming DSS C-API 0.12.
• Fix license (no direct use of EPRI code here); the OpenDSS license is of course still included through DSS C-API, so it's present in the final package.
• Allow using .NET std 2.1 (#if NETSTANDARD2_1_OR_GREATER) to use UTF8 conversion strings; default build is still .NET 4.5 -- users should restrict inputs to ASCII on that.
• Implement GR usage (less memory allocations). See https://github.com/dss-extensions/dss_capi/blob/ccefd69b06dfe09af5daa2d31da7967f14275511/docs/usage.md#global-result-gr-new-in-v0100
• Introduce #if NETSTANDARD2_1_OR_GREATER for UTF8 support. I found .NET support for marshaling UTF8 really crappy before that, it's impressive how long it took them to add it.
• Check for OpenDSS errors for most API calls, mapping them to DSSException.
• Add support for most of the API extensions (properties, toggles and full classes). See https://github.com/dss-extensions/dss_capi/blob/ccefd69b06dfe09af5daa2d31da7967f14275511/docs/known_differences.md
• Update project file
• Drop CmathLib -- not really required in .NET.

TODO:

• [x] Update README.md
• [x] Integrate tags in DLL versioning somehow.
• [x] Ideally, generate HTML help from XML docs (with Sandcastle?).
• [x] Wait for the next DSS C-API 0.12 release
• [x] Reintegrate minimal CI and a simple test / example
• [ ] Provide minimal example with multiple threads

Closes #4, #5, #6, #9, #10. EDIT: prepared (early) preview package: dss_sharp_0.12_preview1.zip