Review how `GWspatialParameters` work

[ccarouge]

For the moment, GWspatialParameters() is called for all simulations so all simulations will try and go read parameters that are only needed when the groundwater is on. It seems it would be best to only call the subroutine when groundwater is on and avoid this subroutine changing values for non-groundwater configurations.

Also, the subroutine tries to read some parameters from file, if it fails, it sets a default for the parameters. This is different from the behaviour for other parameters. The behaviour in CABLE is normally to set the default parameters, then read the input files. The parameters' values for the variables found in the file are updated while the other keep their default allocation.
We need to separate the initialisation to default values for the spatial parameters and the update from file which can stay in GWspatialParameters().
It might be a good idea to set the default values within/called from/called around get_default_params().