Expression: disallow bare functions

[mathause]

As correctly guessed by @veni-vidi-vici-dormivi in #526 (review) I want to disallow passing bare functions to an expression, e.g. loc=mean(). This function would then be translated to loc=np.mean().

@yquilcaille would like to keep this functionality (#526 (comment)).

I still want to remove it because it's not difficult to fix for the user, but difficult to explain and not obvious what happens. Removing also makes the code easier to understand.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.80%. Comparing base (71a1570) to head (20f537e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #531      +/-   ##
==========================================
+ Coverage   61.72%   61.80%   +0.08%     
==========================================
  Files          50       50              
  Lines        3569     3561       -8     
==========================================
- Hits         2203     2201       -2     
+ Misses       1366     1360       -6     

Flag	Coverage Δ
unittests	61.80% <100.00%> (+0.08%)	⬆️

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[yquilcaille]

I don't mind not allowing bare functions, there are certainly good reasons for that ^^
Looks good to me!

[mathause]

Thanks! I think with the error message it should be clear to the user how to fix it.