Migrate to .NET Core 3.0

This topic was initially discussed in #85. When the time comes consider asking @petarpetrovt for assistance/review of the migration and implementation. Migration has been practiced (#95) and should be straightforward.

Wait to migrate until Visual Studio fully supports Windows Forms in .NET Core 3.0: Many demo applications have advanced features (such mouse-draggable axis lines) which are only supported in Windows Forms. Currently, the Visual Studio deisgner does not support Windows Forms in .NET Core 3.0. ScottPlot will conservatively wait to migrate to .NET Core 3.0 until Visual Studio fully supports it.

• consolidate all demos into a single application
• migrate ScottPlot project to .NET Core 3.0 (using System.Drawing.Common)
• ensure target frameworks include net45 and netcoreapp3.0
• add tests for .NET Framework and .NET Core (to detect issues like #138) currently Azure Pipelines does not support running tests in solution files containing mixed .NET Framework and .NET Core projects.
• publish ScottPlot as a multi-framework package on NuGet

[swharden]

Following-up, https://www.nuget.org/packages/ScottPlot.WinForms/ now supports .NET Core 3.0 and .NET Framework 4.6.1

[swharden]

Actually, this may be a good time to consider separating this project into different NuGet packages, each using the framework best suited for the task.

• ScottPlot (.NET Standard 1.3) - plots data and returns or saves an image
• ScottPlot.WinForms (.NET Framework) - interactive user control
• ScottPlot.WPF (.NET Core) - interactive user control
• ScottPlot.UWP (.NET Core) - interactive user control

The primary advantage of this separation is it allows the core function of ScottPlot (creating graphs from data, returning or saving an image) be used in all .NET environments (.NET Framework, .NET Core, Xamarin), on Windows, Linux, iOS, Android, OSX, etc. The secondary advantage is the segregation of interactive (user control) complexity away from the primary plotting library.