Premade forcings in a configured directory

We have prepared several forcings for use in example notebooks. It would be nice if we could have a central location where those forcing could be read from. Similar to parameter set it would be nice if I could do

```python
import ewatercycle.forcing
ewatercycle.forcing.available_forcings()
[
'marrmot-m01_example_1989-1992_buffalo-river',
'marrmot-m01_ERA5_1990-2001_savannah',
'wflow_ERA5_1990-2001_merrimack'
]
forcing = ewatercycle.forcing.load('marrmot-m01_ERA5_1990-2001_savannah')
model = MarrmotM01('2020.11', forcing)
```

I expect `ewatercycle.forcing.load()` method when given a relative path to look in current working directory and if not there also check in `CFG['forcing_root_dir']`

The ewatercycle.yaml could have following key/value:
```yaml
forcing_root_dir: /mnt/data/forcing
```

It could be nice to have multiple `forcing_root_dir` directories to organize things better.