susilehtola and andysim Dft fixes (#1260)
* Better document difference between Psi4 and libxc.

* wPBE0 should only have 100%, not 125% exact exchange at long range.

* Better document beta parameter.

* Maybe this is even better.

* Comment out the two HYB_CAMY and the two HYB_LCY functionals.

* Add note about functionals in future version of libxc.

* Drop unused variable.

* Fix typo from previous commit.

* Improve comment based on comment by Peter Kraus.
Latest commit 7b69f9c Oct 3, 2018