BUG: ICON IVM inst_ids never active at the same time

[jklenzing]

Describe the bug The icon constellation object grabs both IVM objects, but neither exists at the same time. Nominally, inst_id='a' is active, though inst_id='b' is active for campaigns.

To Reproduce

import datetime as dt
import pysat
import pysatNASA

icon = pysat.Constellation(const_module=pysatNASA.constellations.icon)
icon.download(dt.datetime(2020, 1, 1)
icon.load(2020, 1)

Expected behavior Constellation should load.

Additional context This is the result of a poor assumption. Possible routes forward:

• Solve at pysat level to allow loading of instruments without data
• Include multiple instrument lists in the constellation object with a warning that loading by module is not recommended.
• Only keep 'a' in the default list

[aburrell]

One solution may be to have a merge/load function that creates an icon_ivm instrument from 'a' and 'b' combined. Like combining multiple sources of F10.7, for example.

[jklenzing]

This is a pysat bug. Fixed in https://github.com/pysat/pysat/pull/1101

Probably should add a note to the documentation so users know what to expect to close this here.

[jklenzing]

Added examples to the docstrings to show how to correctly load.