Update notebooks

[eoydvin]

When we have solved #29 #37 and #49 we should update the notebooks. My suggestion is to

• remove foo_example.ipynb
• remove openMRG_case.ipynb

For openMRG_case_highlevel.ipynb and OpenRainER_case_highlevel.ipynb we should:

• remove the cell setting radar values below threshold to nan and the cells setting nan to zero, as this task is moved instide mergeplg in Fill nan/radar far away from observations #57
• use poligrain to compute metrics
• Show an example where only rain gauges are used for interpolation (how to construct a grid).
• show multiplicative merging, and how filters introduced in Add range check for IDW, BlockKriging and KED #56 can be applied.

[cchwala]

When you do the updates of the notebooks, I suggest to also add them to the docs and solve #75