ENH: Vasp CSHIFT parameter (missing flag)

The CSHIFT flag is used in the VASP INCAR file to evaluate the dielectric tensor; reduced values are occasionally needed to reduce smearing. The flag was unknown to ASE, so I have added it to the float_keys list. The regression tests are unaffected.

http://cms.mpi.univie.ac.at/vasp/vasp/CSHIFT_complex_shift_in_Kramers_Kronig_transformation.html