Create anisotropy model specification

[jsnel]

Elements of the spec:
base constant (type / enum)
parameters (per compartment)

Create issue for extension c_matrix_gaussian_irf

[jsnel]

Cleaning up the issues list. Questions for you Ivo @ism200 to think about if this is still something for your wish list, or if it's not that important. If so, please add detail to the issue, if not, feel free to close the issue.
In any case I think this will be a post-1.0 feature.