consolidate Mixture with parametric mixtures

[dehann]

Ongoing and further consolidation of factors and different solvers.

FYI @Affie

[dehann]

xref #933

[dehann]

xref #467

[dehann]

There are two kinds of "mixtures" (and therefore actually a confusing name):

• multiple distribution options per same factor mechanics, i.e. Mixture, or
• multiple hypotheses from association, i.e. addFactor!(..., multihypo=[...])

[dehann]

xref #1069