Support multiple instruments in addFromConfig

The addFromConfig config file format only supports a single instrument. The docs say:

# [instrument] <- once and only once with
#   instrument_name  (string)

Including multiple "instrument_name" lines gives an "option already exists" error, which makes sense. But the database structure does support multiple instruments per satellite.

Relation to an issue

Discussion in #78.

Proposed enhancement

Desired behavior needs to be fleshed out further. In particular:

How is this represented in the config file.
How are products mapped to instruments (since this is not currently in the format).

Alternatives

To be fleshed out further.

OS, Python version, and dependency version information:

Linux-4.15.0-143-generic-x86_64-with-Ubuntu-18.04-bionic
sys.version_info(major=2, minor=7, micro=17, releaselevel='final', serial=0)
sqlalchemy=1.1.11

Version of dbprocessing

Branch from #78

Closure condition

This issue should be closed when behavior is fully defined, implemented, tested, and merged.

spacepy / dbprocessing