spin square for FCI wfn

add density fitting HF, DF-MP2

add the missing example uhf_casscf

fix bug in symm for linear molecule. Add symm support for fcidump

import modules in pyscf.future

add mcscf, cc, more fci solver

change cmake layout

fix bug in dhf.direct_scf

version 0.5 update dft, mcscf, etc.

add "future" module for upcoming functions

* module "future" for upcoming functions
* fix bug of AO to MO transformation in OpenMP environment