Discharge [m3/s] for lumped forcing

While looking at #365, I was confused by how the lumped precipitation in mm/s is converted to a discharge in m3/s.

It turns out that in the comparison study, the area is determined using [utils.area.get_catchment_area](https://github.com/eWaterCycle/comparison-study/blob/main/utils/area.py#L8).
The mm/day value of Marrmot-M01 is then converted in the experiment notebook [here](https://github.com/eWaterCycle/comparison-study/blob/18c7b678d6cee0ef1b44be824b509a16973fe4e5/marrmot-m01/experiment_marrmot-m01.ipynb#L285-L290).

I think these kind of utilities would fit better in ewatercycle or the relevant ewatercycle model plugin(s), as it is an essential step if you want to actually analyze the output of a model forced with the DefaultLumpedForcing.

For the LumpedLeakyBucket, this conversion will be required as well.