support inputting Libxc functionals in input file

The goal of this MR is to allow the user to specify Libxc functionals directly in the input, e.g.

&SCF
KSDFT=HYB_GGA_XC_B3LYP

or

KSDFT=2
      1.0 GGA_X_B88
      1.0 GGA_C_LYP

since this is easier than editing the internal database.

• Documentation updated or not needed.
• Test created or not needed.
• Attribution in source files and CONTRIBUTORS.md updated if necessary.