[GL/Grid] Correct grid cell coordinate calculation

The calculation of grid cell coordinates was incorrect. For instance, in case of domains with small extends (<= 1) and equidistant point/node distribution the grid cells are not filled correctly which leads to long run times.

• Run times for identifySubdomains applied to a cube domain discretized by 10^6 hexes in column 2 (without I/O).
• Run times for identifySubdomains applied to a Danube catchment model with 28x10^6 elements in column 3 (without I/O).

average #nodes per grid cell	(averaged) run time cube	(averaged) run time Danube
1	1.64	82.29
2	1.67
4	1.65	86.25
8	1.91
16	1.89	98.42
32	1.93	102.37
64	2.00	132.05
128	1.79
256	2.30
512	2.73