Add support for optional measurements

[caitlinadams]

I'm currently working as part of a team to develop Sentinel-1 backscatter products for indexing into an Open Data Cube.

One challenge of Sentinel-1 is that the polarisation capture mode varies on location, described in the Sentinel-1 documentation under Observation and Production Scenarios. Specifically, a Sentinel-1 observation will have one of the four following measurement combinations:

• VV + VH
• VV
• HH +HV
• HH

This differs from optical products which always have consistent measurements over all datasets.

At this point in time, the ODC makes an assumption that all datasets contain all measurements listed in the product definition. As such, we are currently required to have four separate product definitions, one for each band combination listed above. We were able to circumvent this by trying a product definition that didn't specify any measurements, but it made querying on measurements impossible and was deemed unsuitable for our users.

Ideally, we would like to have one product, which can optionally have any of the measurement combinations, but must have at least one of them. This would allow people to run datacube queries requesting some subset of the above bands and have valid data returned.

As an example, I would be thinking:

• one of HH or VV is required
• all measurements are optional so long as one of the above measurements is present.