the conda docs wrong

CONDA env vars, and tweak $FSLDIR/.condarc so that user ~/.condarc cannot
mess with channel order

dot-separated integers

are baked into the channel URL as env vars. The installer  moves the
channel list from env file into $FSLDIR/.condarc, then sets the env
vars (with credentials obtained from the user) for the installation.

verbose

for regexp

 - All printed messages are also logged.
 - No more SUPPORTED_CUDAS - this is gleaned from the manifest file
 - No need for custom exception types - all errors are propagated directly to
   the user, so the message is much more important
 - When overwriting/replacing, move old directory to tmpdir, then move it
   back if something goes wrong
 - Put fslinstaller.py into $FSLDIR/etc/, for consistency with old install
 - Inform user about update/replace options

manifest to support progress reporting on updates; monitor_progress forces
progress spinner if total output lines not known

code. Forward stream func just uses '' to detect EOF/end process, simpler
this way anyway