Obsolete the complete MathNet.Numerics.IO library

This library is replaced completely with separate libraries for
various data formats and isolated in their own repository and NuGet package.
Among other this allows us to add format-specific dependencies
without any impact on the core library.

Some of the new and already existing packages:

- MathNet.Numerics.Data.Text
- MathNet.Numerics.Data.Matlab
Tests: file rename