Add operator that rejects station data if not complete

### Something else
I believe VER discounts a station if there are any gaps in data for the period of analysis used (exact methodology needs checking). We should have an operator that can be called in verification once we have a model, obs cube, to drop any stations that have data gaps in time. While I don't necessarily think this should be used as standard, having this option would allow us to compare more closely Scores to VER and identify any verification differences due to methodology and not differing data.

Start by locating the verification operators that accept a model and observation cube, then check how station time coverage and gaps are represented. Confirm the exact completeness rule before implementing; done means an optional operator drops stations with data gaps so Scores can be compared with VER methodology.

Written by the indexing model from the issue text.