Footprint data merge for data with multiple inlets

[rt17603]

Based on Issue #77, the output of get_obs_surface is being updated to include details of multiple inlets when these are returned.

Within the code, the presence of multiple inlets returned from get_obs_surface needs to either:

• be interpreted by footprint_data_merge to extract further details from the metadata (e.g. if there are different inlets, make sure they are used align to align each data point with the appropriate footprint)
• OR ensure footprint_data_merge requires an inlet to be specified to get around this.

[rt17603]

In the context of the new ModelScenario way of handling footprint_data_merge - Need to consider how to handle grabbing correct footprints when observation data with multiple inlets has been grabbed from get_obs_surface().

Propose to:

• Call get_footprint multiple times with relevant inlets AND independent date ranges (start_date, end_date).
• As we did with obs data, add a new data variable ("inlet" - check is consistent with obs) containing these inlet values and matching to the time dimension
• Concat this data into one FootprintData object with appropriate metadata data (e.g. attribute and metadata for "inlet" should be "multiple" - again check against obs)
• May want to keep split version and include split obs version for using ModelScenario.footprint_data_merge() method
• Calling ModelScenario.footprint_data_merge() may need to call ModelScenario.combine_obs_footprint() etc. for each inlet to match the right obs and footprint data and resample correctly. May want to concatenate again at the end.

Note: this is only applicable when ranking is in place, otherwise inlet would need to be specified when multiple are available.