Add more bmi variables to wflow

[sverhoeven]

Now RiverRunoff is exposed. It would be nice to expose more bmi variables.

See https://github.com/openstreams/wflow/blob/ad0934ed53519d7ef5793cdd12b72201dd979720/wflow/wf_DynamicFramework.py#L2672 for API config format.

See https://github.com/openstreams/wflow/blob/ad0934ed53519d7ef5793cdd12b72201dd979720/wflow/wflow_sbm.py#L666 for state variables

As workaround edit the config file generated by setup()

import ewatercycle.parametersetdb.config
cfg = ewatercycle.parametersetdb.config.CaseConfigParser()
cfg.read(pcrglob_exp_cfg_file)

cfg.set("API", "SubsurfaceFlow", "2, m/s")

with open('/mnt/home/user30/ewatercycle_output/pcrglobwb_20210830_115514/pcrglobwb_ewatercycle_test.ini', 'w') as f:
    cfg.write(f)

```