Updates to pMincApply and mincApply
1. Bug fix: Added x <- mincGetVoxel in pMincApply.
2. In both mincApply and pMincApply, put x <- mincGetVoxel and test <- eval(function.string) together with comment, to make their purpose clearer.
3. Deleted commented out version of mincApply that is no longer being used.