BUG: TIMED SABER and xarray

[jklenzing]

Description

Addresses #174

TIMED SABER has an extra variable for time, for use with the tpaltitude profiles. This means that 'Epoch' will not be renamed 'time', breaking the new expand_dims code in pysat 3.1.0.

BONUS BUG: when multiple xarray files are combined via xr.combine_nested, the attrs for each variable are dropped, meaning that meta is not set for each variable.

• Renames the profile time variable as 'tp_time', fixing the bug.
• xr.combine_nested now uses the kwarg combine_attrs='override' to use the attrs from the first file.
• Uses the kwarg use_cdflib=True for xarray objects to suppress warning message. pysatCDF does not currently support xarray, so this can be hard wired

Type of change

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

How Has This Been Tested?

Loading a day of data and inspecting the metadata.

import pysat
saber = pysat.Instrument('timed', 'saber', use_header=True)
saber.load(2019, 1)
saber.meta['tp_time']

Test Configuration

• Operating system: Monterrey
• Version number: python 3.9.7
• pysat 3.1.0

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
• Update zenodo.json file for new code contributors

[jklenzing]

Additional meta header issues out of scope, documented as #176

[jklenzing]

This should fix the issues seen in the other PRs