Anisotropy wrapper

[rimadoma]

When coding this branch, I realised that the MILGrid op and ellipsoid fitting didn't fit together. They didn't produce stable results within small tolerance, and input parameter values affected the result too much. This pull request creates a prototype Anisotropy command, that uses a modified MILGrid op.

The modified MILGrid returns the three shortest MIL vectors it sampled on the image. In the command the op is repeated n times and then it tries to fit an ellipsoid on the point cloud that results from these calls. The results change less from run to run than in BoneJ1, and seem intuitive: a cube is very isotropic, a stack of "sheets" parallel to the xy-plane is anisotropic, and "Bat cochlea volume" is somewhere in between.

• Refactors the LineGrid class
• Refactors the MILGrid class
• Adds Anisotropy command
• Adds some tests for the command, and notes on what still needs to be done.