Anisotropic b-factors, do we want them in mmtf?

[josemduarte]

They are missing in v0.2. They would be important for refinement software like phenix. A possibility is that they are treated as user data, but in my opinion they are important enough to deserve their own field.

The field would add a significant amount of information to the file (a 3x3 matrix of floats for each atom), we'd need to decide what's a good compression strategy for it.