random model generation needed for rpa, spherical_sld and onion

[pkienzle]

Hand-tuned random model generators have not been defined for rpa, onion or spherical_sld.

See *random* function in core_multi_shell model definition for an example of how to define random generators for multiplicity models.

[pkienzle]

be_polyelectrolyte and unified_power_Rg need to be checked.

mass_fractal dimension was limited from 1 to 3 since calculations at higher dimensions are not supported across the full q range. Even though the calculation is defined for lower dimensions, the calculation may not be valid for that entire q range. The documentation for the model needs to be updated with a summary of the q limits rather than referring to the reference.

See also the numerical problems which limit the parameter range:

• HayterMSA (#859)
• barbell and capped cylinder (#788)
• binary_hard_sphere (#981)
• paracrystals (#805)