Remove map insert

I've changed two instances of map::insert to map::insert_or_assign, since the former will silently fail if asked to overwrite a value with an existing key. I believe that this behaviour was not intended, since it is impossible to add frames to a dataset via RawDataReader::addFrames and update the PeakCollection objects after processing.

Perhaps it's worth renaming the methods from e.g. addPeakCollection to addOrReplacePeakCollection?