ECC-2256: Add ensemble median type

[mjg41]

Description

In addition to the pre-existing MARS type values "Ensemble mean" and "Ensemble spread" statistics, another possibility for an ensemble is to compute the "Ensemble median". This will be done by the DE330 on-demand use cases for the air quality multi-model ensemble.
To support this use case, we will add the type:

98 emdn Ensemble median

We will also add this to the paramdb with group code 4 - Derived probability products.
We will subsequently add this to metkit.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (faa2dde) to head (0317bbb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #461   +/-   ##
========================================
  Coverage    88.52%   88.52%           
========================================
  Files          849      849           
  Lines        63151    63151           
  Branches     11238    11238           
========================================
  Hits         55906    55906           
  Misses        7245     7245           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.