Uses find_if to perform tower search
Adds a function to sum photons over all clusters
Uses photon sum function in global fit

This allows us to check the number of photons, and quit in case of an
invalid value, once at the start, rather than per-photon in the loop.
Removes superfluous comments

I think this function will be better served with clearer code and
better-named variables
Renames lower and upper limit arrays for fits
Initialises fit arrays with number of photons
Renames variables in global fit for more clarity
Clarifies error messaging in global fit
Updates global fit Doxygen comments
Prevents global fit giving wrong number of photons