static data member is not thread safe and doesn't need to be static

The class SmartPropagator has a non-const static data member that should not be static, for several reasons:
1) it's not thread safe.
2) If the constructor gets called multiple times with different values of the 'epsilon' argument,  the new value will not be used.  This is a bug.  In CMSSW at the moment, it is always called with the same value, but it still is a bug.

This pull request makes the data member non-static.