relax SVN archive

Big bug fix for the RDC and PCS calculations used in the fixed population N-state models.

The last weight, if supplied, was being converted to zero (1 - sum(weights)) hence the last
state/structure was not being included and the population sum was not 1!

Fix for the PCS back calculation for when no weights are given.

Added a check to test_R_to_euler_zyz() for beta angles > pi.

Added a check to test_R_to_euler_zyz() for negative beta angles.

Fix for the test_R_to_euler_zxy() unit test for beta = pi/2.

The alpha and gamma angles subtract in this case.

Fixed the problem case for R_to_euler_zxy().

Removed the beta = -pi/2 checks, as beta is between 0 and pi.

Added more problematic test cases to the euler angle to rotation matrix and back again unit tests.

The problematic beta = -pi/2, pi/2, pi cases.

Added a verbosity arg to pcs.centre() to allow this thing to be silenced!

The PCS centre position can now be a numpy array.