Previously used MGXS_TYPES[:-1] but to be explicit, I instead looped over all MGXS_TYPES and excluded current.
These tests cannot run with current because:
-distribcell is not a valid domain for current
-nuclide/no_nuclides option is not relevant for current
1) Kept by_nuclide, num_nuclides, and nuclides in the docstrings because when building a mgxs library for all mgxs_types, the variables should be the same accross all classes.
However, I noted that they were unused for a SurfaceMGXS.
2) Removed all code related to nuclides and micro xs since they were unused anyway.
3) Removed the possibility of computing a Current with any domain other than mesh.
Included an error message if a current mgxs object is instantiated with any domain other than mesh.
Since mesh domains and by_nuclide features are uncompatible, an existing message is already raised when mesh and by_nuclide are used together.
4) Changed estimator type to analog.
