Add nice error message for splatting in macros

[odow]

Ref #1964. This adds a nice error message, but doesn't enable the functionality, so I haven't gone Closing #1964.

Closes #1964

[mlubin]

The recursive check might be too aggressive. Does it also error at x[some_index_set_generator(args...)]? This should work fine.

[codecov]

Codecov Report

Merging #1977 into master will increase coverage by 0.07%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master    #1977      +/-   ##
==========================================
+ Coverage   88.77%   88.84%   +0.07%     
==========================================
  Files          33       33              
  Lines        4260     4278      +18     
==========================================
+ Hits         3782     3801      +19     
+ Misses        478      477       -1

Impacted Files	Coverage Δ
src/macros.jl	91.37% <95.23%> (+0.29%)	⬆️
src/parse_nlp.jl	90.13% <0%> (+0.65%)	⬆️

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[odow]

Does it also error at x[some_index_set_generator(args...)]?

D'oh. Much simpler...

[mlubin]

To clarify, x[some_index_set_generator(args...)] should not be an error. The current recursive check for ... would error in this case, right? I think the right check is if any of the index sets are exactly splatting expressions, without a recursive dive into the expression.

[mlubin]

I think we should consider this as closing #1964. I'm not compelled to make splatting work. Users are welcome to work around the macros and create their own containers in that situation.