New fixed_format_file class, created by factoring out the general parts of the old t2data_parser class
Added fixed_format_file to list of modules in setup.py
Allowed user-specified conversion functions in fixed_format_file class; expanded docstring
Added t2data support for reading AUTOUGH2 extra precision auxiliary data files
Added t2data support for writing AUTOUGH2 extra precision auxiliary data files
Documentation update for AUTOUGH2 extra-precision data file support in t2data