Enhance epidemic_size()

[pratikunterwegs]

This issue stems from comments by @jamesmbaazam on #176 (below), and requests enhancements to epidemic_size().

• Return a table with demography_group, time, and value;
• Allow returning a range of time points.
• Handle multiple replicates' data as returned by the Ebola model.

• Could you return the "stage" as part of the output? I can easily imagine
  that being useful in cases where a user wants to plot the epidemic size at
  various stages of the outbreak.
• I would suggest renaming "stage" to timepoint or something similar and
  filter by time instead of stage. We often think of epidemic timelines in
  terms of dates and not stages. Using stages forces you to think in terms of
  percentages of the whole timeline. Additionally, you could make "stage" an
  optional argument alongside the new "time" argument.