relax SVN archive

Changes to the linear_constraints() method.

The method is now private and accepts new arguments for the different types of N-state model.

No need to multiply by 1.0 to return a copy at the end - the gradient is reset at the start.

Removed the unused setup_population_eqi() method.

The dipolar constant is now accepted by the RDC functions.

Sent the dipolar constant into the ave_rdc_tensor() function.

The chi-squared gradient is now calculated.

Removed a stray comment.

Created the pc partial derivative part of the RDC gradient.

Wrote the function rdc_tensor() to calculate the standard RDC.

Docstring improvements for the ave_rdc_tensor() function.