Adds the ability to run a forecast in segments instead of all at
once. To accomplish this, the `FHMIN_GFS` and `FHMAX_GFS` settings
have been replaced as user-setable variables in favor of
`FCST_SEGMENTS_STR_GFS`, a comma-separated list of the segment
boundaries (thus there will be one more than the number of segments).
For a traditional single-segment forecast, this would just be set to
`"${FHMIN_GFS},${FHMAX_GFS}"`.

The comma-separated list had to be used instead of a bash array as
the variable must be exported in order for the rocoto generator to
see it, and arrays cannot be exported from shell. Capabilty to parse
these into python lists was added to wxflow in an accompanying PR.

To accomodate the new segment metatasks that must be run serially,
the capability of `create_task()` was expanded to allow a dictionary
key of `is_serial`, which controls whether a metatask is parallel or
serial using pre-existing capability in rocoto. The default when not
given is parallel (i.e. most metatasks).

Resolves NOAA-EMC#2274
Refs NOAA-EMC/wxflow#39

Changes the way forecast segments are defined. Restores the original
`FHMIN_GFS` and `FHMAX_GFS` and then adds a local `breakpnts` variable
that contains the intermediate stopping points (if any). The original
list of segment endpoints is then constructed from that.

The determination of the `FHMIN` and `FHMAX` based on the segment is
moved from `config.base` to `config.fcst`. This required adding some
additional checks in `config.fcst` to clip other `FHMAX` variables to
`FHMAX`.

Updates the extended GFS case to use forecast segments to test that
capability for the GFS system (the GEFS case already tests segments
as well).

Earlier update to parse comma-separated bash variables as lists in
python mean we no longer need to do that in the task scripts.

This commit is incomplete until a follow-up PR that will update the
wxflow hash.

With comma-separated lists now being read into python as lists, the
jinja templates for the archive job had to be updated to not attempt
to create them anymore.

Missed updating one of the dependencies from task to metatask.