Fixes to leaf area and demotion/promotion. Crown area conservation during fusion

This tag contains a set of changes oriented around achieving stable long-term simulations. This includes a feature addition from Charlie Koven that maintains conservation of crown area during cohort fusion. This also includes a set of fixes: 1) demotion and promotion logic, 2) litter fluxes during demotion termination, 3) ensuring minimum patch areas are being imposed, 4) ensuring that fusion and numerical imprecisions to not generate massive LAI via adjusting target plant sizes and 5) reverting to 30 leaf layers and a 2 layer canopy by default. Fixes were implemented by Ryan Knox, Charlie Koven, Jackie Shuman and Greg Lemieux.