2.0.0rc4

This is another update in the ongoing process of moving to specparam 2.0.

This update continues on from rc3, and mostly involves internal updates to re-organize and further generalize the internals of the module to further support multi-mode fitting. Model fitting functionality should not have changed, and at the high level (API of the Model object) procedures should generally be the same or quite similar. Note however that some functions and attributes may have moved internally.