Prevent regeneration of git_version.h by make if not on a new version

* Avoids unnecessary rebuilds.

* Remove "build time" variable
Fix pointer naming in energy functions: intp -> intp_pt
Initial commit of dEdt calculation for magnetostatic energies

* Requires calculation of time derivative of poisson variable

* Possibly not quite right -- errors look a little too big
Add correct (realistic) mu0 value to some magnetic parameter sets