Adds overloaded cluster fit with default steps
Removes default parameters for 2-photon fit

step parameters are always passed, so we will deprecate support for
default step arguments.
Replaces include functional with algorithm

Nothing from functional is used, but std::max() from algorithm is used,
even though the header wasn't included before (we should include
everthing we use, not rely on something else to do so).
Replaces vector push_back with emplace_back

This eliminates a copy operation when adding photons to the vector.
Moves Miniut minimisation to separate function

As we call it the same way in both places it is used, we remove some
duplication of code this way.
Removes check on photon list pointer validity

There are no circumstances under which our code calls the fit function
without a valid pointer list.
Reads Minuit parameters in a function

We do the same thing twice, so it's more maintainable to do it in a
function.
Removes unused Miniut evaluation arguments