allow gaia data_dir to be absolute path

[JoanneBogart]

The update makes it possible to use an absolute path to specify where data files may be found.
To access files at sdf for Gaia dr2, the skyCatalogs config file should contain a section like this:

object_types:
  gaia_star:
    area_partition:
      level: 7
      type: htm
    id_prefix: gaia_dr2_
    data_file_type: fits
    data_dir: /sdf/group/rubin/datasets/refcats/htm/v1/gaia_dr2_20200414
    basename_template: (?P<htm>\d+).fits
    sed_method: use_lut

optionally followed by more object type sections.

[jmeyers314]

LGTM! Thanks for this!