BUG/ENH: sgp4 epoch

[jklenzing]

Description

Closes #67. Fixes a bug where sequential days have a large discontinuity in data when Keplerian elements are used.

Adds epoch kwarg to missions_sgp4 instrument. The epoch is used to specify a datetime that corresponds to the Keplerian elements of the orbit used for propagation.

If a user does not specify an epoch, it defaults to the start of the file range for the instrument. epoch will be set an instantiation.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Using code in #67 and new unit tests.

Test Configuration:

• Operating system: Mac OS 11.6.1
• Version number: python 3.8.11

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 -- fixes for previous pull in same version

[jklenzing]

@rstoneback, there may be a cleaner way of implementing this.

[jklenzing]

Using this to set the default since I can't access self in the load function.

[rstoneback]

Small nit but looks good.

[jklenzing]

Thanks @rstoneback!