Pluralized macros return tuples

[MartinBonde]

Closes #2837
Pluralized macros (e.g. @variables) now return vectors of whatever the 'singular' macros return.

E.g. the following new tests pass:
vars = @variables(model, begin
x
y[1:2]
end)
@test vars == [x, y]
eqs = @Constraints(model, begin
E_x, x == 0
E_y[i=1:2], y[i] == 0
end)
@test eqs == [E_x, E_y]

[mlubin]

Tuples seem like a better choice than vectors in this case. They'll give the compiler a chance at inferring the types of the elements if the situation permits. See https://discourse.julialang.org/t/when-to-use-tuples/18675.

[MartinBonde]

I have no issue with using tuples instead.

[codecov]

Codecov Report

Merging #2844 (7e60976) into master (ee1bc2f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2844   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files          44       44           
  Lines        5695     5698    +3     
=======================================
+ Hits         5398     5401    +3     
  Misses        297      297           

Impacted Files	Coverage Δ
src/macros.jl	95.82% <100.00%> (ø)
src/Containers/DenseAxisArray.jl	90.36% <0.00%> (+0.13%)	⬆️

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[odow]

The documentation is failing because there are a lot of new printing changes now that nothing is no longer returned.

The easiest step is probably to add a semi-colon to the plural usages that appear in the documentation. This is probably a good reason to update the documentation to explain the new return value.

[odow]

Thanks @MartinBonde