Crash or NaN result when convertible cell head equals cell bottom elevation (zero thickness)

[aprovost-usgs]

A zero-divided-by-zero situation arose when the head in a convertible cell was identical to the cell bottom elevation, which should have been identified and adjusted for before the attempted division took place. (See discussion pursuant to commit comment 4fb34f9#commitcomment-125887797.) The issue is fixed in commit 8eca8d8. The corresponding provisional executables are available in the /msvs/bin_PROVISIONAL folder.