removed unnecessary mutables from PixelDigiCollection

The previous code was using std::map::operator[] in places where
it could use std::map::find to get the same results. Switching
to find allowed the removal of the mutables. These mutables were
causing warnings from the static analyzer and would have lead to
crashes in the threaded framework.
Merge pull request #3441 from Dr15Jones/removeUnneededMutableFromPixelDigiCollection

Multithreading -- Removed unnecessary mutables from PixelDigiCollection