MAINT: remove support for meta children and 2d pandas

[jklenzing]

Description

Addresses #789 and #913

Removes the support for storing pandas DataFrames within other DataFrames, including

• the pysat_testing2d instrument
• children in Meta objects

Type of change

• Breaking change (fix or feature that would cause existing functionality
  to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Tested against other pysat packages (ran vs this branch and pysat develop to see if new errors occur)

• pysatCDAAC -- passes
• pysatMadrigal -- no new errors vs develop
• pysatMissions -- passes
• pysatModels -- no new errors vs develop
• pysatNASA -- no new errors vs develop
• pysatSeasons -- passes with changes at MAINT: pysat deprecations pysatSeasons#40
• pysatSpaceWeather -- no new errors vs develop

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 wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release

[aburrell]

Some feedback as an intermediate review.

[aburrell]

_meta.py lines 401 and 442 are newly uncovered, recommend adding an instance where meta is setting bytes instead of a string.

Also have a new uncovered line in io.py on line 1615 that could be addressed with a test for inst_to_netcdf that has a DataFrame of DataFrames to trip the ValueError.

[jklenzing]

_meta.py lines 401 and 442 are newly uncovered, recommend adding an instance where meta is setting bytes instead of a string.

Not sure why these show up as newly uncovered. Neither are covered in develop according to coveralls.

[jklenzing]

Also have a new uncovered line in io.py on line 1615 that could be addressed with a test for inst_to_netcdf that has a DataFrame of DataFrames to trip the ValueError.

Looking at this one, other errors get tripped before we get here since this format is not allowed. I think we can delete this line.

[JonathonMSmith]

There are more lines uncovered that haven't been discussed, but I can't think of straightforward ways to test them

[aburrell]

@JonathonMSmith yeah, there are a few things that could be done, but since there aren't new uncovered lines, I think that's best addressed in another branch.