Locatable event count QA check

[jc-harrison]

FlowETL includes a QA check that counts cell IDs for each day of data ingested, and a check that counts locatable cell IDs (i.e. those that are present in the cells table). This is somewhat useful for detecting if the CDR contains a significant number of unrecognised cell IDs, but even with a large number of unrecognised cell IDs it's quite possible that these only appear in a small proportion of the CDR records. It would be useful to also have a QA check that counts locatable events (i.e events corresponding to known cells).