Changed MagFields verbose::debugOut to constexpr to avoid threading problems

The thread-safey checker complains about the static verbose::debugOut.
Given that no code exists which changes the value after it is initialized
changing this to a constexpr has no functional effect. The default remains
the same 'false'. If one wants the debug output they have to edit the code
and recompile.
Merge pull request #1187 from Dr15Jones/fixStaticsMagneticFieldVerbose

Multithreading fixes -- Changed MagFields verbose::debugOut to constexpr to avoid threading problems