Summer Updates

A few relatively minor changes, mostly centered around xtb/presto integration.

• New PointCharge class which can be added to Gaussian input files.
• fail_silently option enabled for GaussianFile.read_file(), which allows you to check in-progress jobs without a try...catch loop.
• Molecule.csearch() now allows non-GFN2 levels of theory, and can pass command-line arguments to xtb.
• Automatic SI generation with  SIFile.py now allows all .xyz files to be written to a new directory as well.
• Various minor improvements to Gaussian File parsing to handle edge cases.