relax SVN archive

Abstracted the model-free 'classic' Molmol and PyMOL creation in to a base class.

As most of the code between the specific_fns.model_free.molmol and specific_fns.model_free.pymol
modules is identical, this significantly simplifies the code base.

Completely converted the PyMOL model-free macro creation from Molmol to PyMOL commands.

This now reproduces the Molmol 'classic' representation.

The PyMOL macro header has now been converted from Molmol to PyMOL commands.

The menu icons for the pymol.macro_run and molmol.macro_run user functions is now the open icon.

Changed the memory allocation definition for the "curve_type" variable.

It was being defined by an equation involving itself...

Now, the definition assumes a maximum length of 20 characters. This will
be enough for all possibilities, e.g. "exp_2param_neg" has 14 characters,
and "exp_3param_inv_neg" has 18...

Maybe this will again change later.

Fix for the pymol.macro_write back end - the default file extension is now *.pml, not *.mac.

The file name is optional in the pymol.macro_write user function.

Updated the pymol.macro_write user function docstring.

Missing import of relax_io.open_read_file.

Fix for the molmol.macro_run user function GUI page.

The directory argument should have been None.