BUG/STY: update platform names

[jklenzing]

Description

Addresses #49

With the release of pysat 3.0.0, 'pysat' is reserved for platforms from the core package. Updates platform names to 'missions'.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How Has This Been Tested?

Tested by registering the instrument package and importing assuming registration.

import pysat
import pysatMissions
pysat.utils.registry.register_by_module(pysatMissions.instruments)
sgp4 = pysat.Instrument('missions', 'sgp4')

Test Configuration:

• Mac OS X 10.15.7
• python 3.8.2
• pysat 3.0.0

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