More vibration options

I've tried to update the current implementation of Vibrations to be for flexible so that you can run forward difference without calculating the backwards stencil element for central difference. I also cleaned the stencil code so that it supports nfree=4 forward and backwards difference options properly. All of the functions attached to the Vibrations class should now default to the Vibrations.method and Vibrations.direction strings rather than overriding them. All of this is in support of building the ability to build Hessians by progressively adding elements to them. This feature is very handy for large DFT systems where you might be interested only in the local VDOS. This is especially useful in harmonic transition state theory calculations of the attempt prefactor which can converge in as few as 20 atoms.