OPT:
  - fixed the case where input images are not in orientation RPI (sct_extract_metric) adding a new function in sct_utils to change image orientation
  - add two new functions in sct_utils to find a file in a directory tree from the exact file name or from a patter in order to remove hardcoding to find the vertebral labelling file in sct_extract_metric
  - fixed the case where several slices include two different vertebral levels (sct_extract_metric)