Add Data Assimilation

As part of my Master Thesis with @RolfHut I'll be working on a framework to run data assimilation on eWaterCycle.
My final usecase is the use Particle Filters to assimilate streamflow in the [HBV](https://github.com/Daafip/ewatercycle-hbv) model in the CAMELS catchments. But the goal is to have it working such that any type of Data Assimilation can be added.

There has been some experimenting [in the past](https://github.com/eWaterCycle/streamingDataAssimilation), but more to show that is it is possible.

Looking at the requirements of the different Data Assimilation methods currently used (PF, EnKF, ES, ES-MDA, IES ), I designed the following work flow:
Which I've started implementing [here](https://github.com/Daafip/eWaterCycle-DA) (still experimental currently, but running).

My main question is where should be integrate this into eWaterCycle?

From a disucssion today with @RolfHut & @BSchilperoort we thought of adding it as an entry point to ewatercycle.util?
This way users see it as an extra untility function.

Curious to hear your thoughts on this!