AP_RangeFinder: TeraRanger a to close reading is not the minimum distance

[WickedShell]

This is a bit of a drive by change, but as far as I can tell using the minimum distance here is unsafe. The problem with copying the minimum distance in, is that if we are to close to get a reading we could be any distance less then the minimum distance. However the logic for determing if a rangefinder is good or not only considers the driver to be out of range low, if the distance is less then the minimum distance. Logic can be seen here.

This could be combined with the following case, I kept them separate just to ensure that the meanings are preserved, and to make it easier to directly flag something like OutOfRangeLow in the future.

I don't actually have one of these units, so I can't test (or even verify) if this is a real problem (and solution). Does anyone have the hardware to actually do the test?

[rmackay9]

The problem with this solution is that the lidar will show "unhealthy" instead of "out-of-range-low". Instead I think we should probably return 0 as the distance so its state becomes "out-of-range-low". A similar argument is also true for when the distance is too long.

Certainly this PR is an improvement over the previous logic which was dangerous though.