Turned all the tests into classes
Restructured some things

Just doing a bit of a tidy up so some public interfaces are a bit neater
Changed ConstitutiveModel interface

Hopefully it makes a little more sense now?
Added TerminationCriterion::cancelled()
Moved some classes around

Moved Sim::ForceFieldSet to ForceField::ForceFieldSet, and did the same
with Sim::CollisionObjectSet