relax SVN archive

Added a RelaxError for when the PDB MODEL record is corrupted.

The RDC and PCS error data structures are now properly handled and setup in __init__().

The RDC and PCS errors data structures are now created by the __minimise_setup_*() methods.

These are also passed into the minimisation code.

Try 2: PCS errors in a file separate from the data can now be read.

This matches the changes for the RDC at r7202, and is what r7204 should have been.

Reverted the last revision, r7204, as this included too much.

The command used was:
svn merge -r7204:7203 .

PCS errors in a file separate from the data can now be read.

This matches the changes for the RDC at r7202.

Allowed the PCS data column to be None, so that a file with only PCS errors can be read.

RDC errors can now be read separately and independently from the data values.

Allowed the data column to be None in the rdc.read() user function.

This allows a file containing only errors to be read.

Added a print out to the centre() function.