relax SVN archive

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.

The calculated paramagnetic centre is now being placed into the current data pipe.

Added a missing import and fixed an incorrect reference to the data pipe container.

Shifted the calculation of the paramagnetic centre from __minimise_setup_pcs() to centre().

The data pipe arg is now accepted by the centre() function.