P3 estimator is the only variance estimator supported by mrds

Currently we warn with

```
Warning message:
In dht(model, obs.table = analysis$env$obs.table, sample.table = analysis$env$sample.table,  :
  Point transect encounter rate variance can only use estimator P3, switching to this estimator.
```

(which comes from `mrds`). Unclear if Distance for Windows implements other estimators.