Unify profile fitting API

[thclark]

The function API for fit_power_law_speed passes parameters directly, and does not allow variation of u_ref as a parameter, instead simply fitting alpha.

• Alter the cost residual to accept u_ref and z_ref as parameters. NB problem is underconstrained if you allow them both to vary, so ensure there's an assertion to allow only one of them (either fit alpha and u_ref, alpha alone, or alpha and z_ref).
• Alter the API to accept an Array2d &initial_parameters and Array2b &fixed_parameters the same as the API for fit_lewkowiz_speed. It may be possible to generalise the cost functor used for fitting lewkowicz speed, to help keep code DRY.
• Update docs and unit tests correspondingly.