MAINT: Future Warnings for numpy, pandas, pysat

[jklenzing]

Description

Updates usage based on future warnings raised in testing

• For Series data, use ffill and bfill instead of fillna with a method.
• Use .iloc for position data
• Updates to pandas frequency strings
• Usage of use_header in pysat

Type of change

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

How Has This Been Tested?

via pytest and inspecting messages.

Test Configuration:

• Operating system: Mac 14.5
• Version number: Python 3.10.9
• pysat branch from BUG: Assumptions in clean tests pysat#1200 (fixes a bug in 3.2.0 for pysatNASA)

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