Add examples and tests for epinowcast methods and uncovered plots

[seabbs]

This PR adds basic examples and tests for summary.epinowcast(), plot.epinowcast(), and enw_plot_obs() (which was the only plotting function not fully covered by the new tests for plot.epinowcast()).

It also resolves an issue with the default in enw_plot_quantiles which was TRUE vs FALSE as documented.

[codecov]

Codecov Report

Merging #209 (f6cebff) into develop (2b06668) will increase coverage by 5.89%.
The diff coverage is n/a.

❗ Current head f6cebff differs from pull request most recent head 2dc3b7b. Consider uploading reports for the commit 2dc3b7b to get more accurate results

@@             Coverage Diff             @@
##           develop     #209      +/-   ##
===========================================
+ Coverage    88.20%   94.10%   +5.89%     
===========================================
  Files           14       14              
  Lines         1492     1492              
===========================================
+ Hits          1316     1404      +88     
+ Misses         176       88      -88     

Impacted Files	Coverage Δ
R/methods.R	100.00% <ø> (+46.66%)	⬆️
R/plot.R	91.66% <ø> (+91.66%)	⬆️

... and 2 files with indirect coverage changes

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[pearsonca]

Seems generally fine, but also: this should be its own extension package.

[seabbs]

@pearsonca do you have a review decision on this? Going to merge in #208 first, resolve merge issues, and then merge this in once ready.

[pearsonca]

Comfortable with it. Added issue(+) for splitting off vis package + introspecting Qs.