Compute MGH volume orientation string.



Header of the mgh datastructure, as returned by read.fs.mgh.


character string of length 3, one uppercase letter per axis. Each of the three position is a letter from the alphabet: `LRISAP?`. The meaning is `L` for left, `R` for right, `I` for inferior, `S` for superior, `P` for posterior, `A` for anterior. If the direction cannot be computed, all three characters are `?` for unknown. Of course, each axis (`L/R`, `I/S`, `A/P`) is only represented once in the string.