Correct incorrectly calculating min and max cell for each piece.

Previously we incorrectly calculated the min and max cell
for each piece if we had less cells that pieces. This was
caused by using integer division instead of float division.

Also I correct the issue that if we have zero cells we would
divide by zero causing VTK to crash.

Change-Id: Id7f99dfa0b6b85727f326a38c62ada16af8a51ac
Merge topic 'update_extract_ugrid_piece' into master

8a73bfeb Correct incorrectly calculating min and max cell for each piece.