Silence compiler warnings

[mattjala]

Mostly dealing with issues around the const qualifier. Some changes of note:

• The data in field for RV_parse_callback now has the const qualifier, so most callbacks now have a const qualifier on their data in field as well.
• Removed the use of a callback in RV_multi_perform, now just checks the transfer type and uses the read callback directly. Removed the empty write callback.
• Because the get_link_val_callback needed to modify the size of the in buffer,the size of the in buffer was paired with the buffer itself in a new struct, get_link_val_out.
• Renamed RV_parse_type -> RV_parse_object_class to distinguish it from RV_parse_datataype

[jhendersonHDF]

This works (if a bit awkward), although we could probably get away with just making path and cur_pos const while using a different var. than cur_pos when indexing into tmp_buffer.