relax SVN archive

The n_state_model.select_model() user function can now change the model.

Previously a RelaxError was thrown, but now a warning is given that the model is changing. This
allows the 'population' N-state model to be optimised by first optimising the 'fixed' model and then
switching to 'population'.

Big bug fixes for the _minimise_setup_pcs() and _minimise_setup_rdcs() methods.

This allows the missing data PCS and RDC optimisation system test to pass. Many problems were
solved to allow all types of data with anything missing (one spin in one alignment, one spin in all
RDC or all PCS lists, or entire RDC or PCS lists for one alignment).

Bug fix for the __det_pdb_element() method.

RNA PDB files with missing element information is now handled properly. The stripping of the "'"
was being ignored.

Removed a debugging print statement.

Fixes for the structure.load_spins() user function docstring for RNA/DNA work.

Modified the dna_pcs_fit.py N-state model system test script to use the truncated PDB file.

This speeds the system test up by orders of magnitude.

Created a truncated version of the LE.pdb file for the RDC and PCS system tests.

This contains just the atoms for PCS and RDCs, and will significantly speed up the tests.

Loosened a value check in the test_pcs_fit_zero_pos N-state model system test.

This was sometimes failing, but the value was correct enough.

Created the test_missing_data N-state model system test.

This should pull out a few hard to find bugs.

Wrote a script for testing the missing RDC and PCS data.

This uses the N-state model with N=1, so direct testing of the single state to 4 tensors.