Segmentation fault in VectorFieldSimulator

[pietercollins]

VectorFieldSimulator yields a segmentation_fault after completing a long simulation run.

[pietercollins]

The problem is due to the use of Real as the current time. Since this has a symbolic representation, successively incrementing yields a deeper and deeper dependency tree, which causes a stack overload on deallocation. Fixed by changing to FloatDPApproximation.