Cross-check clino on diving data and average

[Olly Betts]

1.2.20 will allow clino/backclino readings in diving style data.

However, the current implementation just checks these readings for syntactic validity, and otherwise ignores them.

It ought to attempt to detect blunders by cross checking them with each other (when both clino and backclino are specified), and against the depth and tape readings.

And the leg ought to use averaged data from the redundant measurements to improve accuracy.