Splits in mapmaker

[paganol]

This PR contains a first attempt of introducing data and time splits in the mapmakers.

• [x] Implement the support for splits in binner.py
• [x] Fix tests for the binner
• [x] Define splits
• [x] Add high level functions for automatically loop over make_binned_map
• [x] Add documentation

[ggalloni]

I enhanced the current loop over splits by moving it into the sims framework to access write_healpix_map. In such a way, we can save to disk each map to disk without having to allocate all of them. Still, there is a flag switching back to the dictionary if desired by the user. Also, I added some docs on the available splits within the function that check their validity.

Before committing, however, we may want to address an "issue" I am getting with ruff: running ruff format ., it does not complain of anything. Instead, when committing it shows extra requests to fix stuff related to formatting. I followed its suggestions for the previous commits, however, it started being quite annoying... Do you have any idea why it is doing this?

[ggalloni]

Let me explain the ruff problem (now fixed) so that we can keep track of it: if ruff is not pointed explicitly to a configuration file, it will search for the nearest pyproject.toml containing a section [tool.ruff]. The issue is that it will search for it system-wise, meaning that will pick up any other file with those specifications (such as another project...).

Thus, it was actually sufficient to add that section in the .toml of litebird_sim to fix the issue. I also added an empty extend-select which may be useful in the future if we want to include extra ruling on the format.