Creates new microstructural plot

Does a few other things as well:

1. minor changes to the FA calculation, so it now gives 0 instead of nan for isotropic signals and nan instead of random numbers for the dot compartment.
2. fixed the computation of cylinder and tensor compartments, so that the microstructural parameters can be arrays
3. added isotropic property to all Compartment classes, which is True if the compartment does not depend on "bvec". This is now used to speed up the calculation of the spherical mean and the ODF convolutions.
4. Moved the calculation of spherical mean from the plotting code to the compartment code. This is based on the spherical mean signal of the total signal being equal to the sum of the spherical means of the individual compartments (this is not true for the MD/FA, which are still calculated in the plotting code).

The new plots seem to work for all parameters of the currently implemented compartments

Closes #94 (closed)