Diffgamma

Added a null implementation for diffgamma.. the meat of it has to be done with either

1. physical approximation - may prove difficult
2. large gamma grid - will prove a pain in the ass

README

+This is a project based on DFTATOM (Certik 2013) and edited by Levämäki
+(2015) for orbital-free calculations. The idea of the project here is to
+add the necessary gamma-dependent terms.
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