Warn if surveyed date is before explored date

[Olly Betts]

This seems a useful warning, but the check really needs to be at the point where the dates are actually applied to a leg which means we need to actually store the explored date which currently we don't.

Also we need to set a flag to say we've warned which gets cleared when either date is changed - otherwise we'll warn for every affected leg, which would potentially be a lot of essentially duplicate warnings so unhelpful.

It's probably OK if one date is less precise and they overlap as a result, e.g.:

*date explored 2004-08
*date surveyed 2004-08-20

(This could even adjust the explored range to be 2004-08-01 to 2004-08-20.)

Internally that's the same as:

*date explored 2004-08-01 2004-08-31
*date surveyed 2004-08-20

That means this is most easily handled in the same way, which is probably OK even if we could warn.

Or perhaps we could warn for both (since either could be tightened to *date explored 2004-08-01 2004-08-20).