Add timestep manipulation routines

[oiffrig]

Description

Add utility functions to manipulate time steps and step ranges

Documentation

• User's guide: https://earthkit-time.readthedocs.io/feature-timesteps/guide/api.html#manipulating-time-steps-and-ranges
• API reference: https://earthkit-time.readthedocs.io/feature-timesteps/api/timesteps.html

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

❌ Patch coverage is 78.16092% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.26%. Comparing base (ee199f1) to head (0222e53).

Files with missing lines	Patch %	Lines
src/earthkit/time/timesteps.py	66.27%	57 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #13      +/-   ##
===========================================
+ Coverage    61.33%   64.26%   +2.92%     
===========================================
  Files           24       26       +2     
  Lines         1239     1500     +261     
===========================================
+ Hits           760      964     +204     
- Misses         479      536      +57     

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