consistent naming: params

[aaronpeikert]

I seperate this issue out of #236 since it is quite involved.

• params should not longer return Vector[Symbol] but return values
• param_values should be hence renamed and first suggestion was paramnames as a replacement for the old function
• however there are other params functions such as: params_indices, and param_occourences, param_refs, should it be paramnames or param_names?
• params is used as argument a couple of times, replace it with param(_)names there as well
• params should work on AbstractSem not only on ParameterTable
• the column param in parameter table must also be renamed
• lavaan uses label so I think my preference would be param_label for the replacement function to be consistent with other functions and lavaan.

[aaronpeikert]

closed for #239