allow substrate csvs to omit voxels and entries in rows

[drbergman]

Currently, every voxel needs to be assigned a substrate concentration for every substrate column. Any substrates without a column are initialized as 0 everywhere, i.e., they ignore the XML.

Now, this PR allows for omitting voxels and having missing values in a row. Any missing voxel or missing value are set to 0. Thus,

x,y,z,sub1,sub2
20,0,0,1,
0,20,0,,1

1. initializes sub1 to 0 everywhere except (20,0,0) where it is 1
2. initializes sub2 to 0 everywhere except (0,20,0) where it is 1

Any other substrate would be 0 everywhere.