Several important FFEA scripts throw exception on normal usage

[Robert Welch]

• The script FFEA_get_vdw_area references the vdw.calc_active_areas method, which doesn't exist as far as I can see.
• The script FFEA_make_bsites_vdwactive references the FFEA_binding_sites object, which can only be created by the FFEA_convert_vdw_to_bsite script. This script references the FFEA_vdw.vdw_index attribute, which does not exist.
• The cull_small_interior_elements script calls sys.exit() right in the middle. If this is removed, it references FFEA_element_tet_sec.get_volume, which also does not exist.

I suspect that these scripts have not been updated for a while, but they are part of the list of scripts that Ben gave me to packagize. I'm happy to assign this bug to myself, but I have no idea how to fix them, so I would appreciate a nudge in the right direction.

[Robert Welch]

These scripts are old and are all due for deletion.