platform/processingLevel concept-id aliasing, DATASET Constants, constants no longer top level imports

[SpicyGarlicAlbacoreRoll]

Changed

• Constants are no longer top level import, are now accessible through respective modules
• search keywords processingLevel and platform are now aliased by collection concept-ids when available, improving performance and potentially dodging param splitting subquery system (lists of concept ids by their processing levels/platforms viewable in dataset.py).

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1efbbe0) 89.98% compared to head (927d621) 90.06%.

Files	Patch %	Lines
asf_search/search/search_generator.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #238 +/- ## ========================================== + Coverage 89.98% 90.06% +0.08% ========================================== Files 54 54 Lines 2046 2073 +27 ========================================== + Hits 1841 1867 +26 - Misses 205 206 +1 ``` | [Flag](https://app.codecov.io/gh/asfadmin/Discovery-asf_search/pull/238/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=asfadmin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/asfadmin/Discovery-asf_search/pull/238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=asfadmin) | `90.06% <98.59%> (+0.08%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=asfadmin#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.