Move Madrigal index Instruments to their own sub-modules

[aburrell]

Description

Addresses #91 by creating instrument sub-modules for the remaining madrigal index files.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality or documentation)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

import pysat
import pysatMadrigal as pymad

dst = pysat.Instrument(inst_module=pymad.instruments.madrigal_dst, user='User Name', password='your email')
dst.download(start=pymad.instruments.madrigal_dst.madrigal_start)
dst.load(1981, 1)

# Repeat for the madrigal_geoind and omni2_imf Instruments

Test Configuration

• Operating system: OS X Big Sur
• Version number: Python 3.9
• Any details about your local setup that are relevant

Checklist:

• [x] Make sure you are merging into the develop (not main) branch
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have linted the files updated in this pull request
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] Add a note to CHANGELOG.md, summarizing the changes

If this is a release PR, replace the first item of the above checklist with the release checklist on the pysat wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release