libvdwxc: more functionals
Add the GGA functionals offered by old vdw.py to the libvdwxc interface. Now missing only MGGA-BEEF. One change affects the rest of the code: a new get_description() function on XCFunctional which by default returns None but can return a description printed by output.py. This fixes the problem of using the 'name' property both as an identifier and a description.