Generates forcings when requested.

The notebook generated by /pyewatercycle endpoint (#29) expects the forcings to be inside pararmeter set or already prepped in `/mnt/data/forcing/` directory. It would be nice if the endpoint could be passed properties which would generate forcings.

For wflow the prop could look like
```js
forcing_options: {
dataset:"ERA5",
start_time: "1990-01-01T00:00:00Z",
end_time: "1990-12-31T00:00:00Z",
shape: "/Merrimack.shp",
model_specific_options: {
"dem_file": /staticmaps/wflow_dem.map"),
},
}
```

There are some problems to overcome:
* map model.name to target_model. Propably with lower()
* have central place for shape files so relatives paths can be used in Terria catalog item
* have flexible definition of `model_specific_options` that works for supported models
* dem_file can be found in parameter set or some where else. Need to decide which relative path to pass.
* lisflood forcing generation still needs manual steps, see https://github.com/eWaterCycle/ewatercycle-lisflood/issues/11