Make neighbourhood radius in wind-direction CLI configurable

[rachaelesler]

(Raised by Bureau of Meteorology)

As a Research Scientist
I want to increase the neighbourhood radius value when calculating mean wind direction from ensemble realization
So that I can smooth out spurious local variability in IMPROVER outputs for wind direction

As a Research Scientist
I want to decrease the neighbourhood radius value when calculating mean wind direction from ensemble realization
So that I can preserve more spatial detail in IMPROVER wind direction outputs

In the file improver/wind_calculations/wind_direction.py, the neighbourhood radius is currently hardcoded to 6,000 metres. In practice, in the IMPROVER Suite at the Bureau of Meteorology, a greater neighbourhood radius is desirable to produce stabler, "smoother" forecasts for wind direction.

Adding another optional argument to the wind-direction CLI, would be a good solution to this, e.g.

improver wind-direction --nb-radius 10000 <input-cube> 

The neighbourhood radius would take a default value of 6,000 so that all existing uses of the wind-direction CLI will be unaffected.

Note: the NeighbourhoodProcessing() plugin that the wind-direction CLI depends on already accepts an argument for the neighbourhood radius, making this a simple feature to add and test.

Acceptance criteria:

• A neighbourhood radius argument can be passed to the wind-direction CLI
• If no neighbourhood radius argument is passed to the wind-direction CLI, the code behaves the same as before
• The default neighbourhood radius is documented
• The argument accepts a positive floating point value