Test features

[malerany]

Implement a test to ensure that the code is working properly for each of the placeholder files added to tests/placeholders.

Assignees: @malerany

Migrated from GitLab: https://gitlab.earth.bsc.es/ces/hanami/pyhanami/-/issues/9

[malerany]

I created a new branch to track the development of the tests: features/tests

For now, I started adding .txt files as placeholders for the tests within the folder tests/placeholders.

I am using the following template for the .txt files:

# Test Plan: <Feature or function name>

## Purpose
Briefly describe what this test is supposed to verify

## Inputs to test
Describe input data or conditions to be tested:
  - Normal case
  - Edge cases (e.g., empty dataset, missing variables)
  - Invalid inputs (e.g., wrong data type)

## Expected behaviour
Describe what the correct behaviour/output should be in each case (including any files/plots that should be created):
  - For valid inputs
  - For edge cases
  - For invalid inputs

## Status
- [ ] Not Started (placeholder)
- [ ] In Progress (corresponding .py script in progress)
- [ ] Ready for Implementation (ready to merge into `develop` branch)
- [ ] Completed (tested with corresponding functions in `develop` branch)

[malerany]

I already added the placeholders for the functions in utils/statistics.py, encompassing together in one file the functions that compute metrics for the replicability test.

[malerany]

I created the placeholder for testing the regridding functions (defined in utils/data.py) and added input validation to the functions in utils/statistics.py.

[malerany]

I created placeholders for testing the methods in DataDiagnostics() related to spatial plots and their dependencies in utils/data.py and utils/statistics.py.