Port AMR Reductions

[julianakwan]

This would implement the GRChombo feature of calculating the L2 norms of the Hamiltonian and Momentum constraints and outputting them to a separate constraints file via SmallDataIO - see example here

I also propose changing the naming of the parameter responsible (calculate_constraint_norms) because this is very confusing as there is also a variable nameda_calc_mom_norm variable passed to the set_up function in the Constraints class.