Improved output of initial state

[dietervdb-meteo]

This PR closes #61 and #48.
A new function reduce() is added to the Output class, which projects a state to the last step in the multi-step dimension.

This function is then used to define write_initial_state() as a combination of reduce() and write_state() for the netcdf, raw and plot output.

The naming of the raw output is brought in line with the naming of the plot output.

The default write_initial_state config setting is now true.

---

📚 Documentation preview 📚: https://anemoi-inference--62.org.readthedocs.build/en/62/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.00%. Comparing base (ecfb2bc) to head (1f1ec4d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #62   +/-   ##
========================================
  Coverage    75.00%   75.00%           
========================================
  Files            1        1           
  Lines            4        4           
========================================
  Hits             3        3           
  Misses           1        1           

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

[dietervdb-meteo]

Questions:

1. Currently default config is not to write the intial state to output. Shouldn't it be better if the default is to write the initial state?
2. Plot output does the projection to last step of in the multi-step dimension explicitly (and for all states, not just the initial state). This could now be done using reduce() and possibly applied only in write_initial_state. Should we change this?

[dietervdb-meteo]

Upon (private) communication with the reviewer it was decided the answer to both questions above is 'yes'. The code was adapted accordingly.