Model skeletons

[lentinj]

The last point from https://github.com/gadget-framework/gadget3/issues/48:

• Model skeletons (is this a good idea? ):
  • g3_single_stock, define a single stock with all the "usual" bells and whistles
  • g3_two_stocks, define a immature and mature stocks with all the "usual" bells and whistles

I think they're a good idea, and vastly easier now thanks to all the g3_parameterized() defaults.

[lentinj]

Superseded by https://github.com/gadget-framework/gadget3/issues/118