Removed mutable from DigiCollectionFP420

Removed mutables from DigiCollectionFP420 by using std::map::find
instead of std::map::operator[]. The code in question was not dependent
upon the default value being placed in the map, it only wanted to
see if a value was available. This avoids warning messages from the
static analyzer.
Merge pull request #3440 from Dr15Jones/removeUnneededMutableFromDigiCollectionFP420

Multithreading -- Removed mutable from DigiCollectionFP420