docs: fixes for unit support and other minor corrections

[ManuelHu]

• rename fccd param to fccd_in_mm
• read ak array with units in the tutorial and make units a bit more explicit.
• we should iterate a bit on the docs and how to "mark" unit-aware parameters. I got some questions from users because of the lack of docs on this topic.

some other things I found:

• HITS.__edep should not work since a long time ago (did it ever work?), that should now only be HITS.edep in a config file.

[tdixon97]

I think it looks good, I agree with you we have a bit more work still to do on unit handling.

I think we should gradually phase out the config files approach which is a bit hard to maintain / test. But I still think we need to provide some "ready to use" post-processing options to not require everyone to write their own scripts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.04%. Comparing base (d31e919) to head (7a4f723).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   70.04%   70.04%           
=======================================
  Files          33       33           
  Lines        2597     2597           
=======================================
  Hits         1819     1819           
  Misses        778      778           

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