Add a "none" grid type

[mdpiper]

This is a request to add a new BMI grid type for a variable that could have multiple values, but doesn't have grid information associated with it.

For example, I'd like to list a GDAL GeoTransform as an output variable. This is a fairly well-defined format--it's an array of six float values. But what grid type should be used? It seems like "vector" or perhaps "unstructured" could be used, but the GeoTransform doesn't have any grid associated with it, it's just an array of values; so, e.g., get_grid_x wouldn't be defined, but get_grid_rank and get_grid_shape could be.

[mdpiper]

The "none" grid type is accepted in the bmi-tester, so perhaps this request reduces to adding "none" to the BMI documentation.

[mdpiper]

Also allow get_var_location to return "none". This may simply be documentation issue. See #61.