Parameter Space: allow marginal parameters and fixed parameters

[lzkelley]

Currently all parameters going into Param_Space classes become new dimensions of the parameter space.

• (1) Add the ability to add a parameter that is samples from / marginalized over, but formally not an additional dimension --- just a nuisance parameter.
• (2) allow parameters to be set to fixed values, so that everything can be set in the same place and manner, again without adding a new dimension to the parameter space.

Presumably these could both be handled in a very similar way.

[lzkelley]

Implemented in combination of _Param_Space and PS_Generic, but it should be re-structured after the 15yr

[lzkelley]

Closing this, and considering this as part of API improvement for gen_lib_sams (etc)