relax SVN archive

Small fixes and improvements to the rdc.back_calc() user function.

Align IDs are being used instead of RDC IDs so RDCs can be back-calculated even if measured RDCs are
not present. And spin info is also being checked for better error reporting to the user.

The Ln3+ position optimisation system test has been sped up by decreasing the accuracy.

Fix for the alignment tensor initialisation for when the ID is added to cdp.align_ids.

This was introduced in r11301. cdp.align_ids was not being checked if the align ID already exists.

Added some unit tests for the pseudo-elliptic cosine function.

Added a system test for the back-calculation of PCSs.

Added a severely truncated PDB file for PCS back-calculation with ubiquitin.

Wrote the pcs.back_calc() user function back end.

Initialising and alignment tensor now adds the ID to the alignment ID list.

Fix for the alignment tensor rotation matrix - it is now always in the right handed universe.

Added a system test checking the conversion of alignment tensor to chi tensor.