adding solvent to bilayer builder
check if unit_cell lengths are specified in to_compound
set unit_cell.lenghts if they are not given in trajectory.to_compound
added ability to include multiple components in bilayer builder
make bilayer class save lipids and solvent, and changes to how the layers are mirrored
Merge pull request #40 from tcmoore3/bilayer

adding solvent to bilayer builder
Merge branch 'develop' into bilayer
resolving conflict in solvent_box