Subregions of arbitrary shape

[lang-m]

• add numpy array (?) of int (subregion_index) with subregion index + dict (?) mapping labels to integers (subregion_)
• shape of that array must match mesh.n
• initialise array like field::value and field::norm
• keep support for old subregions dictionary
• automatically translate legacy subregions into new array
• no support to access subfields based on subregion labels defined outside the dictionary (show example in the documentation instead)
• add support in mumax3c and later oommfc
• review plotting routines