Fitting Models with a random subset: clunky syntax

[Jerome Fung]

Specifying a Model to fit to a random subset of a hologram currently requires passing an ImageSchema to the schema_overlay kwarg in the Model constructor:

 model = Model(par_s, Mie.calc_holo,
                  schema_overlay = ImageSchema(use_random_fraction = 0.02,
                                               origin = tgt_small.origin),
                  alpha = par(0.75))

(see lines 47--50 of the attached script).

This is both clunky (consider a use_random_fraction kwarg to the Model constructor), and also requires re-specifying the origin if we're fitting to a subimaged target hologram.