bilayer initialization with tilt

[chrisiacovella]

Currently, generation of a bilayer sets bilayer cross-sectional area based on calculating the "bounding box" of the lipids in the leaflet. Thus, when lipids are long and highly tilted, the actual area per lipid is significantly larger than the nominal value set. Furthermore, this void space is increased upon mirroring the leaflet; for my tests with CER AP, this results in unstable bilayers.

I'm working on a fix in my branch.