Export ∆ (\increment) and ∑ (\sum) as aliases of Laplacian() and sum()

[wsshin]

This PR exports ∆ and ∑ as aliases of Laplacian() and sum().

Note that these symbols are entered by \increment and \sum, and different from Δ (\Delta) and Σ (\Sigma) that are more commonly used as variable names. Therefore, exporting these symbols do not clutter the namespace.

This PR fixes #760. Once this PR is merged, the documentation mentioning Δ (\Delta) as an alias for Laplacian() should be updated with ∆ (\increment).

[codecov]

Codecov Report

Merging #761 (2561b30) into master (fef0c7f) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
+ Coverage   30.28%   30.40%   +0.12%     
==========================================
  Files          15       15              
  Lines         809      809              
==========================================
+ Hits          245      246       +1     
+ Misses        564      563       -1     

Impacted Files	Coverage Δ
src/Extras/poetry.jl	6.25% <ø> (+6.25%)	⬆️

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[dlfivefifty]

Note this is a breaking change so you should bump the minor version

[jishnub]

The visual similarity between \sum and \Sigma would make code using both hard to read. Maybe this shouldn't be exported? Users may import this if their projects aren't likely to use both.