Cache physical variables in wvm

[JeffreyEarly]

This would prevent (or at least mitigate against) repeated calculation of the same variable for multiple purposes. The cache would then get hosed when new coefficients are set.... urg, doesn't work. The problem is that the physical variables depend on time and only the integrator has the concept of time.

[JeffreyEarly]

One way to achieve this is to add observationTime or modelTime to the WVM. So, it will cache variables at the obsTime, but not for other times.