MEP -> CNEMC

[lewisblake]

Change Summary

All references to MEP have been changed to CNEMC as this is the new name for the network.

Related issue number

closes #1176

Checklist

• [x] Start with a draft-PR
• [x] The PR title is a good summary of the changes
• [x] PR is set to AeroTools and a tentative milestone
• [x] Documentation reflects the changes where applicable
• [x] Tests for the changes exist where applicable
• [x] Tests pass locally
• With the exception to test_load_berlin as described in this PR
• [x] Tests pass on CI
• with the exception of code coverage which is not relevant for this PR
• [x] At least 1 reviewer is selected
• [x] Make PR ready to review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.31%. Comparing base (ac80b76) to head (6e100b5). Report is 30 commits behind head on main-dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main-dev #1202 +/- ## ============================================ - Coverage 79.31% 79.31% -0.01% ============================================ Files 131 131 Lines 20230 20231 +1 ============================================ Hits 16046 16046 - Misses 4184 4185 +1 ``` | [Flag](https://app.codecov.io/gh/metno/pyaerocom/pull/1202/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/metno/pyaerocom/pull/1202/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | `79.31% <100.00%> (-0.01%)` | :arrow_down: | 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=metno#carryforward-flags-in-the-pull-request-comment) to find out more.

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[thorbjoernl]

Should the following lines in paths.ini also be changed?

L136: MEP = MEP L176: MEP = 2013

[lewisblake]

@jgriesfeller my vote is to keep the file path in paths.ini the same until the download location on the CAMS Validation Server changes, and then we change the entire name in the AEROCOM_OBSDATA database.