Added letters parameter to mulgrid rectangular(), refine(), split_column(), add_layers(), from_gmsh(), refine_layers() methods- to allow user-specified set of letters to use for naming
Bugfix in from_gmsh()- introduced by new letters parameter
Changed defaults for mulgrid rectangular() case and letters parameters- now (deprecated) case parameter defaults to None, and letters defaults to ascii_lowercase (which should have the same effect- but means the default for letters is more consistent across different mulgrid methods)
Added documentation of new letters parameter in various mulgrid methods
Renamed mulgrids function IntToLetters() to int_to_chars()- can now use characters that are not letters
Changed name of new letters parameter in mulgrid rectangular() and other methods to chars, and updated documentation accordingly
Added TOUGHREACT support to t2incon class
Added documentation of new TOUGHREACT support in t2incon class