ENH: Overhaul xc flag of Vasp calculator, implement many common xc functionals

Issue #20 (closed) contains extensive discussion of this proposal.

The xc parameter formerly specified the VASP pseudopotential (POTCAR) set. This is confusing as while selecting the POTCAR does also set a default GGA, there are many desirable XC functionals that do not have a designated POTCAR file (only LDA, PW91 and PBE are implemented).

This merge requests implements a new parameter pp determining the origin of the POTCAR files to be used, and the xc flag is adopted to set multiple parameters from a dictionary. The dictionary Vasp.xc_defaults is an attribute of the Vasp calculator class and may easily be modified by the end user; it is hoped that additional standard "recipes" will be contributed over time.

Still to do: Write tests Update documentation