nbhood_land_and_sea handling of missing data

[thbom001]

There appears to be an inconsistency in the way the nbhood_land_and_sea plugin handles missing values in the incoming data cubes. Basically, missing values in the incoming data (for example a whole field of missing values when the model data did not get created for some reason) get treated as "0" values in the nbhood calculations. This appears to be inconsistent with the way other parts of the IMPROVER code handle missing values - the missing values get propagated through as missing values rather than being changed to something else.

I believe I have identified and fixed the code which causes this issue, and will create a PR and update this issue after I've done some further testing of the potential fix.

Acceptance criteria:

• Missing data gets propagated through nbhood_land_and_sea processing and remains as missing in the output.