handle grid type scalar

[sverhoeven]

In

grpc4bmi/grpc4bmi/reserve.py

Line 33 in 6a05c8a

if model_type in {'uniform_rectilinear', 'rectilinear'}:

we now handle 'uniform_rectilinear', 'rectilinear', but there is also 'scalar' (https://bmi.readthedocs.io/en/stable/#get-grid-type).

Scalar looks like the right type to use for lumped models.

A lumped model might not even know where (geographically) is is being run.

We should figure out what the get_grid_* functions should return for a model with get_grid_type=='scalar'.