First attempt at implementing the paramagnetic centre position gradient in the N-state model.
This will be used for faster optimisation of the lanthanide position. Two new functions have been
added:
maths_fns.pcs.ave_pcs_tensor_ddeltaij_dc()
maths_fns.pcs.pcs_constant_grad()
These are used by the dfunc_*() methods of the N-state model target function class.
