Redo determinant enumeration: optimise for truncated CI.
Redo determinant enumeration: optimise for truncated CI.

We can greatly improve (ie make accessible) truncated CI calculations
where the full Hilbert space is vast by only enumerating determinants
within the truncated space rather than enumerating the full space and
testing whether a determinant is within the truncated space.

Even on a tiny system (4x4 square lattice with 6 electrons) a CISD
calculation on my netbook now takes 0.05s rather than 0.35s...

hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande