Add variable broadcasting for expressions

[benkrikler]

This PR adds the functionality of variable broadcasting before passing into numexpr.
This means we can now mix variables with different jaggedness more conveniently within expressions, with less need for external stages.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@072cc2f). Click here to learn what that means.
The diff coverage is 95.45%.

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  Coverage          ?   65.97%           
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  Files             ?       20           
  Lines             ?     1487           
  Branches          ?        0           
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  Hits              ?      981           
  Misses            ?      506           
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Impacted Files	Coverage Δ
fast_carpenter/expressions.py	94.73% <95.34%> (ø)
fast_carpenter/version.py	100.00% <100.00%> (ø)
fast_carpenter/backends/alphatwirl.py	21.42% <0.00%> (ø)
fast_carpenter/define/reductions.py	75.64% <0.00%> (ø)
fast_carpenter/summary/import_aghast.py	57.14% <0.00%> (ø)
fast_carpenter/__main__.py	0.00% <0.00%> (ø)
fast_carpenter/help.py	0.00% <0.00%> (ø)
fast_carpenter/summary/binning_config.py	80.00% <0.00%> (ø)
fast_carpenter/define/systematics.py	89.13% <0.00%> (ø)
... and 12 more