Work on convert library to .Net Core

[ygrenier]

Work on create some projects to migrate PCL Library to .Net Core Library.

• Convert SwissEphNet Library
• Convert SweMini program
• Convert SweTest program

SweWin can't be converted to .Net Core.

[ygrenier]

The package is available as prerelease: https://www.nuget.org/packages/SwissEphNet/2.5.1.14-beta-1

It contains the Library for:

• .Net 4.0
• PCL profile 136
• .Net standard 1.3

[ygrenier]

Because actually we can't mix .csproj/.xproj, the program "SweWin" can't reference SwissEphNet and be correctly compiled.

So for the moment we used the Nuget package for this program.