Rename package import and xarray accessor from `gpm_api` to `gpm`

[ghiggi]

Prework

• [x] I understand and agree to this repository's code of conduct.
• [x] I understand and agree to this repository's contributing guidelines.
• [x] I have already submitted an issue or discussion thread to discuss my idea with the maintainers.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Documentation
• [ ] Tutorial
• [ ] Code style update
• [x] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [x] Yes
• [ ] No

If yes, please describe the impact and communicate accordingly:

This PR introduce several BIG breaking changes.

• It modify the way on how users imports the software. From: import gpm_api to import gpm

• It modify the name of the gpm_api xarray accessor. Example: From ds.gpm_api.plot_map() to ds.gpm.plot_map()

• It modify the name of the config file. From config_gpm_api.yml to config_gpm.yaml

  The PR fulfills these requirements:

• [x] It's submitted to the branch named as follow :

  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Improve a tutorial tutorial-<some_key>-<word>
  • Add a new feature: feature-<some_key>-<word>
  • Refactor some code: refactor-<some_key>-<word>
  • Optimize some code: optimize-<some_key>-<word>

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

• [ ] Don't forget to link PR to issue if you are solving one.

• [ ] All tests are passing.

• [ ] New/updated tests are included

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.65121% with 65 lines in your changes are missing coverage. Please review.

Project coverage is 81.75%. Comparing base (8bd40e5) to head (d611a32). Report is 14 commits behind head on main.

Files	Patch %	Lines
gpm/accessor/methods.py	74.64%	18 Missing :warning:
gpm/utils/archive.py	0.00%	9 Missing :warning:
gpm/utils/collocation.py	12.50%	7 Missing :warning:
gpm/utils/geospatial.py	54.54%	5 Missing :warning:
gpm/retrievals/retrieval_2a_radar.py	0.00%	4 Missing :warning:
gpm/retrievals/routines.py	20.00%	4 Missing :warning:
gpm/utils/manipulations.py	0.00%	4 Missing :warning:
gpm/dataset/decoding/routines.py	25.00%	3 Missing :warning:
gpm/visualization/profile.py	57.14%	3 Missing :warning:
gpm/visualization/title.py	0.00%	2 Missing :warning:
... and 6 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #42 +/- ## =========================================== + Coverage 62.97% 81.75% +18.78% =========================================== Files 63 103 +40 Lines 5912 9783 +3871 =========================================== + Hits 3723 7998 +4275 + Misses 2189 1785 -404 ```

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