Merge clusters connected components

This merge request implements:

1. Function depositedEnergy() to DigitizationCell
   => This is needed because in order to allow an energy threshold also for analogue readout
   => depending on the user implementation this can be calculated differently
2. A function which merges cells at the same position (also summs up energy deposition) (using boost::connected_components)
3. A function which merges clusters of cells which either share common edge or common edge and corner (using boost::connected_components)
4. Unit tests for the above functions

Fixes ACTS-453.