Speed up driver __init__ by caching exceptions.

[bas-rustenburg]

This is a reminder of the slow parts of init, in case we ever want to speed it up. We may be able to cache some exception parameters instead of retrieving them for every titratable group.

Relevant code statements

protons/protons/driver.py

Line 595 in 43b9734

group['exception_indices'] = self._get14exceptions(self.system, atom_indices)

protons/protons/driver.py

Line 374 in 43b9734

[particle1, particle2, chargeProd, sigma, epsilon] = force.getExceptionParameters(exception_index)

Not a priority.

[jchodera]

It's definitely a good idea to cache those.