docs: def pkg_available(pkg_name,required=False):
      '''tests if analysis package is available on this machine (e.g., "afni" or "fsl"), and prints an error if ``required``'''
      if pkg_name in pkgs:
          return True
      if required:
          nl.notify('Error: could not find required analysis package %s' % pkg_name,level=nl.level.error)
      return False