STY: use standard init routines

[jklenzing]

Description

Addresses #121

• Moves acknowledgement string and references to methods files for all (non-deprecated) instruments.
• Uses standard init routine for other methods
• Updates references for new calibration of ISS FPMU data

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Instantiating instruments and printing refs:

import pysat
fpmu = pysat.Instrument('iss', 'fpmu')
print(fpmu.references)

Test Configuration:

• Operating system: Monterrey
• Version number: Python 3.9.7

Checklist:

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

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

[aburrell]

Most changelog comments seem to use past tense, so updated to be consistent with that.