MAINT: backward compatibility for pysat

[jklenzing]

Description

Addresses #112, #114, #123

The requirements file no has a minimum version of pysat 3.0.4.

• Removing if/else statements and tests for older version of pysat.
• Update location of load_netcdf

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

pytest

Test Configuration:

• Operating system: Monterrey
• Version number: Python 3.10.9

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

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