relax SVN archive

Fix for the __update_model() method.

This was only working if both RDC and PCS data are supplied.

A call is now made to assemble the PCS data necessary for optimisation.

Changed the way optimisation of the N-state model is set up.

This will allow multiple base data types to be used simultaneously.

Added a module docstring.

Fixed all the calls to __base_data_types() and subsequent use of the returned list.

Fixed __base_data_types() to properly identify tensors as the base data source.

Modified the __update_model() to handle both RDCs and PCSs.

Redesigned __base_data_types() to determine all base data types, and return them as a list.

Renamed __determine_data_type() to __base_data_type().

Created 2 new RelaxErrors for PCSs.