For completeness

Before:
    AssertionError: True is not false

After:
    AssertionError: True is not false : file "/tmp/tmp4oox0ycu/avhrr-only-v2.20190528.nc" exists, but should not
    AssertionError: True is not false : file "/tmp/tmp4oox0ycu/avhrr-only-v2.20190527.nc" does not exist, but should

The filename was present earlier in the stack, but call it out for
clarity.

While both are logically an inability to write, they have different
errnos. Python's PermissionError encompassed EACCES and EPERM, but not
EROFS.  Ticket: #372

issue #10

We encouraged setting CSPP_GEO_AITF_ANCIL_TIMEOUT to a large value as a
workaround, so we can't rely on it being a good value.

https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/345

Also add some _getenv tests, including for the new functionality.

Rely on a non-routable IP address. Hypothetically might fail if some
layer is configured to return errors for such addresses. But it's a lot
easier that standing up (and maybe suspending?) a mini server.

Resolves
https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/345

Standardize help messages between aitf-data-for-run and aitf-update-cache https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/298

We need default of _2_ days cache since we start TOMORROW https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/298

Clarify --expiration is "unused" files, not "old" https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/298

Was removed over a month ago; just failed to remove obsolete
documentation

it was causing a test grief, also it looked bad