Add mcif reader

[rebeccafair]

From #97 (comment).

This is a feature request to add a reader for the mcif format, which is an extension of the widely used Crystallographic Information File (cif format) to handle magnetic structures.

Unlike CIF which is a standard of the International Union of Crystallography (IUCr) the mcif format was created for an online database (the "ISOTROPY" suite) but is now fairly widely used.

There is a reader implementation in pymatgen (docs; code).