Finalise the exact exchange implementation

[davidbowler]

Exact exchange is implemented but not yet integrated into the code. There is some refactoring of the ground state search required to implement this in the most efficient way (this will be partly covered by work done in issue #23 ). The work will take different stages:

1. Post-hoc EXX calculation
2. SCF EXX calculation
3. Forces due to EXX terms

Part 1 is almost finished; part 2 requires the refactoring; part 3 is partly done.