Finds the FSLDIR from system environment or getOption("fsl.path")
for location of FSL functions
Usage
get.fsl(add_bin = TRUE)
Arguments
add_bin
Should bin be added to the fsl path?
All executables are assumed to be in FSLDIR/bin/. If not, and
add_bin = FALSE, they will be assumed to be in FSLDIR/.
Value
NULL if FSL in path, or bash code for setting up FSL DIR