Morphologies in miniapp (#178)

Fix morphology section ctor bug.
Add morphology pools for miniapp from which morphologies are drawn in the recipe.
Add command-line options to expose above.
Add option --report-compartments to (slowly) check the min, mean, and max number of compartments in the generated cells across the simulation.
Use morphology::add_section to do all the heavy lifting; no need to make section_geometry objects by hand, unless you really, really want to.