Simplify the 'single-sensor-per-dataset' aggregation rules

[mwengren]

This relates to the concept of the observing 'Platform' as described in the Platform Section of the profile.

From the Profile:

single-sensor-per-dataset: individual ERDDAP datasets of either TimeSeries or TimeSeriesProfile type that are associated via matching platform_id and wmo_platform_code global attributes, breaking a platform’s data apart into individual DSG datasets. Some IOOS RAs use this pattern already, and provide one dataset for sensor package (e.g., CTD, ADCP, met, etc). The code used to ingest sensor datasets for GTS by NDBC or for Sensor Map by IOOS will perform this aggregation, so data providers need not create aggregations in ERDDAP. Each individual dataset intended for the GTS must follow the Requirements for NDBC/GTS Ingest individually.

Note that because neither platform_id or wmo_platform_code attributes are required in this profile, any dataset without them will be considered to be specifying data for only one, distinct platform, and aggregation with other datasets by downstream processes at NDBC or IOOS is not guaranteed.

Allowing aggregation of these 'single-sensor-per-dataset' datasets based on either platform_id or wmo_platform_code creates an avoidable burden on downstream data processors (e.g. NDBC, Axiom/Sensor Map), without any apparent benefit.

Suggestion: simplify the aggregation rules (and above docs) to only imply these datasets will be aggregated by platform_id if provided. Otherwise, no aggregation will occur. Remove wmo_platform_code from the above.

Any comments/concerns with this change?