Using new fixed_format_file class to read mulgrids: implemented for reading nodes
Use fixed_format_file class to read header; also corrected an error in fixed_format_file docstring
Use fixed_format_file class to write header and nodes
Use fixed_format_file class to read and write columns
Use fixed_format_file class to read and write connections; fixed bugs in read_nodes() and read_columns()
Use fixed_format_file class to read and write layers
Use fixed_format_file class to read and write surface
Use fixed_format_file class to read and write wells
Merge branch 'fixed_format_mulgrid' into testing