Suppress warnings about test XML documentation

 - Add 'nullable' to style cop dictionary.
Rename 'ToMaybe' conversion methods

 - Rename tests class.
Make 'ToMaybe' an extension method

 - Remove temporary implicit conversion operator. This didn't work
perfectly in the area of Nullable types when they didn't have a value,
so better to avoid this altogether and stick with the 'ToMaybe'
extension method.
Rename project to YAM