Update cochran depth precision: it's in 3-inch increments

Update cochran depth precision: it's in 3-inch increments

The Cochran CSV depth exports are indeed in tenths of feet, but the
decimal is always 0, 3, 5 or 8.  Where the 3 and 8 are obviously 0.25
and 0.75 rounded up to one decimal place.

So Cochran does seem to be very much about imperial units, with depth
and cylinder pressure scaled by four (depth in quarter-foot increments,
pressume in 4-psi increments)

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>