sbig-snap: enable auto-freeze

If TE regulation is enabled, set autofreeze flag so that
regulation is quiet during readout.  It is essential that we complete
the readout - now that SIGINT is handled we can be pretty sure.

Fixes issue #7
sbig-cooler: remove inappropriate modes
sbig-snap: get filter from cfw configuration, if used

If filter is set to "cfw" in the config file, then query
the filter position from the cfw and read the description of
that position for the FILTER FITS header value.

Fixes #4