relax SVN archive

Pre-calculated the dipolar constants prior to optimisation.

Wrote the dipolar_constant() function for calculating the dipolar constant.

The constant is defined as:

mu0 gI.gS.h_bar
d = - --- -----------
4pi r**3

Added a missing import.

Fixed the N-state model code to use the renamed averaged RDC functions.

Wrote the average_rdc_tensor_dDij_dAmn() function.

This calculates the ensemble averaged RDC gradient element for the Amn partial derivative.

More rewriting of the population N-state model optimisation code.

Large expansion of the dfunc_population() docstring.

Fixed some index names.

Fixed a call to the non-existant chi2_element() function (should be to chi2()).

Missing import.