Add support for .NET Standard

[hanwavefront]

Love the library, but I'm getting the following warnings when I include it as a dependency in my netstandard2.0 project:

warning NU1701: Package 'C5 2.3.0.1' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.
warning NU1701: Package 'TDigest 1.0.8' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.

Functionality doesn't seem to be affected, but I would like to see .NET Standard support added to the library to avoid the warnings.

[ByronComparisonTech]

Is this library still active or has it been sunsetted?