The most important part of this commit is the inclusion of tests for

some LDA functionals. The calculated value of these functionals is 
compared to reference data that can be found in 
http://www.cse.clrc.ac.uk/qcg/dft/.

Note that the tests do give some errors. This is related to fully  
polarized cases, i.e., when rho_down=0. The reference data hard-wires, 
in this case, the value of vpot_down=0, which, in my opinion, is not 
correct. In any case, if the gas is fully polarized, the value of 
vpot_down is irrelevant in this case ;)

There were some very small precision issues in a couple of functionals 
that I adjusted in order to match the reference data.

In the process I added two new LDA "functionals": 
  *) lda_c_pz_mod. This is the version of the Perdew Zunger functional 
modified by Alberto to match better the low and high rs regions of the 
parametrization. This is the old pz that we had in the code. The 
original PZ functional is called lda_c_pz. Note that the default is the 
corrected functional.
  *) lda_c_vwn_rpa. This is a parametrization of the RPA correlation 
energy using the VWN form, and that is also present in the original 
paper.

Soon I will start playing with the GGAs ;)