Better non-rectilinear grid support

In https://github.com/eWaterCycle/pywflow/blob/cf44beb2e4dceb5e1bd794e50145a1bf1b8c6f55/src/ewatercycle_wflowjl/model.py#L34 and https://github.com/eWaterCycle/pywflow/blob/cf44beb2e4dceb5e1bd794e50145a1bf1b8c6f55/src/ewatercycle_wflowjl/model.py#L238 better support was added for non-rectilinear grids. However this is only usefull in the wflowjl plugin. It would be nice to move those functions to https://github.com/eWaterCycle/ewatercycle/blob/main/src/ewatercycle/base/model.py .

We will need to add unit tests as wflowjl plugin only has notebooks.