Add autoplot and fortify methods for poolaccum

[codecov-io]

Codecov Report

Merging #11 into master will not change coverage. The diff coverage is 0%.

@@         Coverage Diff          @@
##           master   #11   +/-   ##
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  Coverage       0%    0%           
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  Files          20    22    +2     
  Lines         534   554   +20     
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- Misses        534   554   +20

Impacted Files	Coverage Δ
R/autoplot.poolaccum.R	0% <0%> (ø)
R/fortify.poolaccum.R	0% <0%> (ø)

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[gavinsimpson]

Thanks for the contribution @delferts.

I tweaked the fortify method, mainly for style and to have the Index variable first in the returned DF.

I largely rewrote the autoplot method:

• used geom_ribbon for the quantile interval — I prefer ribbons for intervals in ggplot, with alpha transparency
• didn't hard code colours; would prefer to inherit whatever colour scheme is in effect or allow user to over-ride this by adding scale_xxx_yyy as needed, without having to do the entire plot by hand.