Add order of hourly calculations to For contributors

[mabijkerk]

There is a specific order in the hourly calculations:

        @fever.calculate_frame(frame)
        @heat_network.calculate_frame(frame)
        merit_calc.call(frame)
        hydrogen_calc.call(frame)
        agriculture_heat_calc.call(frame)

This order is shown in Atlas and there are comments about in spread throughout the code - perhaps in ETEngine as well? Because it is very relevant for contributors to know, we should add documentation about it in the For contributors section. Better to have concise documentation, then no documentation at all.

@louispt1 I think it would be good if you could pick this up at some point (with input from @noracato and me).

[noracato]

Hey! This is not part of Atlas, but part of the Causality plugin in ETEngine :)