Update VIIRS AOD converter to work with NASA Dark Target and Deep Blue products

[weiwilliam]

Description

This PR makes follow updates:

1. Use the original NOAA data reading code as a function.
2. Add two functions for NASA Dark Target and Deep Blue products.
3. Remove the argument to mask out _FillValue data, always keeps valid points now.
4. Update the dateTime to seconds from epoch time.
5. Add an optional argument of date_range to process data within the specified window.
6. Updated list of arguments:
   Required: --input, --output, --provider [nasa/noaa]
   Optional: --retrieval_method [DarkTarget/DeepBlue], --error_method [pue], -n (--thin), --date_range

Observation error selection:
Pixel-level uncertainties estimates (PUE) are recommended for DA purposes based on section 3.1 VIIRS DeepBlue paper.

For NOAA product, the PUE is surface-dependent error statistics with considering solar zenith angle and sensor viewing angle, so I modified the original "nesdis" method to "pue".

For NASA Dark Target, I could not find papers about the PUE, so the expected error (EE) is assigned as default.

For NASA Deep Blue, the PUE are two variables for the retrieval over land or ocean.
I currently assign the PUE to obs error based on the number of land and ocean points used in retrieval.
In the case of data retrieved with land and ocean points together,

• The number of land points is more, use the PUE over land and vice versa.
• By any chance the numbers of land and ocean points are equal, use the average of PUE.
  Additionally, for Deep Blue retrieved with land and ocean points, the PreQC value uses the poorer quality when the numbers of land and ocean points are equal.

Issue(s) addressed

Resolves #1563

Dependencies

List the other PRs that this PR is dependent on:

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Impact

Expected impact on downstream repositories:

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have run the unit tests before creating the PR

[CoryMartin-NOAA]

Please merge in develop and then I think this can be approved