Updated GaussianBasisSet documentation regarding the shell->center map.
Fixed a couple of leaks
Started work on invsqrt-including DF transforms ... not implemented though, and may not be pending TA breakthrough
Reapplied changes undone to ShellPairsLibint2 (copy ctor) in a54fee31d86664d9b47e55170346f4aa90d3e105 . Closes #18
Fixed remaining typo for shellpairs.cc
Merge branch 'master' of github.com:ValeevGroup/mpqc into localdf