Support for mutable classes #136

Currently the Spatial library provides immutable structures which are great for direct geometry or for including in other entities. I don't think this should change.

However there is another use case where the geometry is the primary entity. Where the geometry is involved in motion and needs to be updated many times a second and is associated with additional data. In this case it would be more helpful if the geometric items could be created once and be modifiable.

I suggest therefore to add another set of classes - I propose mathnet.Spatial.MutableGeometry namespace which are freezable, mutable classes which support external reference data via a Tag property and nested geometric entities

What do you think? Should this kind of use be supported in this library?