Dynamically allocate cell data arrays

[rouson]

Rationale

The original version of TCelSim fixes the number of cells as the compile-time constant value ncells. In parallel execution, the set of cells will be distributed across images. Because the number of images will not necessarily be known at compile time, we need to allow for the number of cells in a given image to be determined at runtime. For this reason, arrays that have ncells as one dimension's extent need to be dynamically allocated.

Steps

• Add the allocatable attribute to the following array declarations in the main program: random_number_table, x, y, and z.
• Add an allocate statement in the main program for each of the above arrays