default number_of_units for non-investable and investable units

[nnhjy]

Fixes #1163

Checklist before merging

• Documentation is up-to-date
• Unit tests have been added/updated accordingly
• Code has been formatted according to SpineOpt's style
• Unit tests pass

[nhniina]

Looks in general ok to me. I'm just wondering why to use

        + (
            is_candidate(unit=u) ? 
            number_of_units(m; unit=u, stochastic_scenario=s, t=t, _default=0) : 
            number_of_units(m; unit=u, stochastic_scenario=s, t=t)
        )

instead of creating _default_nb_of_units(u) = is_candidate(unit=u) ? 0 : 1 and then using that in number_of_units(m; unit=u, stochastic_scenario=s, t=t, _default=_default_nb_of_units(u))? Wouldn't it be cleaner? I didn't quite understand how that is related to whether or not units have units_on (#1163 (comment)).

[nnhjy]

Looks in general ok to me. I'm just wondering why to use

        + (
            is_candidate(unit=u) ? 
            number_of_units(m; unit=u, stochastic_scenario=s, t=t, _default=0) : 
            number_of_units(m; unit=u, stochastic_scenario=s, t=t)
        )

instead of creating _default_nb_of_units(u) = is_candidate(unit=u) ? 0 : 1 and then using that in number_of_units(m; unit=u, stochastic_scenario=s, t=t, _default=_default_nb_of_units(u))? Wouldn't it be cleaner? I didn't quite understand how that is related to whether or not units have units_on (#1163 (comment)).

Good suggestion! Thx. They are implemented in the new commit.
I've also updated #1163 (comment) based on the latest progress.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.38%. Comparing base (c143931) to head (9448056).

Files with missing lines	Patch %	Lines
src/constraints/constraint_common.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1184      +/-   ##
==========================================
- Coverage   87.38%   87.38%   -0.01%     
==========================================
  Files         144      144              
  Lines        4423     4430       +7     
==========================================
+ Hits         3865     3871       +6     
- Misses        558      559       +1     

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