reduce computational cost #32

ideas to reduce computational cost:

1. during shell minimizations, compare energies with loose convergence criteria to eliminate high energy conformers earlier in the process (don't run high energy conformers all the way down hill as they will be discarded anyways)
2. only run saddle point searches on low energy conformers after shell optimization (instead of trying to find saddle points for all conformers)

• generate conformers --> prune with DFTB (low level) --> prune with DFT shell opt (high level) --> run saddle point searched on remaining conformers