Adapt optimisation methods so they work with monthly data

[wouterpeere]

Currently, all the optimisation methods only work with hourly data. However, in the back-end, the optimise_load_energy functionality already calculates with a monthly load.

An idea to make this transition possible is to work with a load-duration-curve that a users inputs. Doing so, we can convert our methods to be able to work with monthly loads where, instead of using the convert_hourly_to_monthly method, we use the load-duration-curve to convert our loads.

Accuracy of this assumption will have to be researched thoroughly.