Divide the SOILWAT2 output into cell-specific files

[chaukap]

This is a part of the larger issue #467. Right now the SOILWAT2 output is all contained in the same files in the following format for a 4 x 4 grid:

cell 0 day 1
cell 1 day 1
cell 2 day 1
cell 3 day 1
cell 0 day 2
cell 1 day 2
. . .
cell 2 day 365
cell 3 day 365

Obviously it would be ideal if we could break this input up into 4 files. Because SOILWAT2 has no idea that gridded mode exists, and the last thing I want to do is couple SOILWAT2 and STEPWAT2 more tightly together, I think the best course of action is to create a function that divides the output files from SOILWAT2 into M files where M is the number of cells.