plot random set of models, including uncertainty

just like the function that's still in bdfit.BDSampler, picks a
random set of models to plot, and plots them, including uncertainty.
Splitting to a new module allows plotting without rerunning the fit,
and allows other plotting codes to access it.

Eventually I might want to make this universal enough that it can
work for any kind of model. Maybe.
don't adjust subplots, a sub-gridspec instance doesn't do it.  Also get current figure instance
return random samples, update var names and plotting

return the random samples so they can be used by other programs
(in future, should probably make calculating the random models
its own function, so that I don't have to run this first)

updated variable name random_sample->random_samp because random_sample
is already a function or module in numpy

changed some labels and fontsizes for plotting
Plot random sample on corner plot; kludgy plot adjustments for now

using subplots_adjust to change subplot pars, which isn't great
because the corner plot needs different pars than the model
plot, but oh well.

Kludgy way of dealing with ndim and burn_in, eventually make that
more universal