MeshDomain caches MeshFunctions, therefore will not reflect changes in MeshValueCollections

[Garth Wells]

The MeshDomains class holds MeshValueCollections to denote mesh (sub-)domains. It has a convenience function that converts a MeshValueCollection into a MeshFunction, and then caches the MeshFunction. If the MeshValueCollection is changed, the MeshFunction will not reflect this change.

Simplest fix is to not cache MeshFunction, and make the caller responsible for managing efficient re-use. This is another example of why caching should generally be avoided.