Removed unneeded const_cast

Although the const_cast was not needed, it was showing up as a warning
by the thread-safety checker.
Changed statics to const statics

Previously, the function local statics were not declared const because
there was no way to easily set their values via the constructor. This
code was changed to use 'const function statics' where we use a helper
function to do the setup. This avoids thread-safety issues.
Merge pull request #1304 from Dr15Jones/threadSafeDataFormatsDTRecHit

Multithreading fixes -- Thread safe DataFormats/DTRecHit