potential performance updates

[ffreyer]

I'll update this whenever I find something.

• In local_updates, rather than calculating log(exp_delta_S_boson) the action difference could be saved to a variable and the log could be dropped.
• eVop1, eVop2 and eVop1eVop2 are only actively used in calc_detratio as far as I can tell. Having just interaction_matrix_exp_op!(mc, new_op - p.hsfield[:,i,s.current_slice], +1., s.eVop1eVop2) should be enough.
• Mtmp, Mtmp2 are only used in detratio. Mtmp2 could be removed if mul!(s.Mtmp2, s.delta_i, s.Mtmp) becomes lmul!(s.delta_i, s.Mtmp). M should also be fine to use instead of Mtmp.

[carstenbauer]

Yes. But note that this is irrelevant on the time scale of update_greens! which follows this line.