add plot_bivariate() function

[cmahony]

This function is now complete and ready for merge into devl.

[CeresBarros]

looks great!

[CeresBarros]

@cmahony there's a test failure related to the plot_bivariate example. png(..., type = "cairo") is not readily supported in macOS systems.
I'll need to set up the GitHub Actions node differently to support PNG form macOD

Also, there's a missing import in plot_bivariate and undeclared variables -- I can fix.

[CeresBarros]

note to @cmahony : pushed several fixes. Waiting for checks to pass then, if all is good, will merge :)

[codecov-commenter]

Codecov Report

Attention: 125 lines in your changes are missing coverage. Please review.

Comparison is base (9037362) 84.60% compared to head (9596219) 78.57%.
Report is 11 commits behind head on devl.

❗ Current head 9596219 differs from pull request most recent head 859b035. Consider uploading reports for the commit 859b035 to get more accurate results

Files	Patch %	Lines
R/plot_bivariate.R	0.00%	105 Missing ⚠️
R/historic.R	87.14%	9 Missing ⚠️
R/gcm.R	93.22%	8 Missing ⚠️
R/normal.R	94.73%	2 Missing ⚠️
R/database.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             devl     #200      +/-   ##
==========================================
- Coverage   84.60%   78.57%   -6.04%     
==========================================
  Files          21       22       +1     
  Lines        1637     1806     +169     
==========================================
+ Hits         1385     1419      +34     
- Misses        252      387     +135     

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[CeresBarros]

merged but see #203