feat: Revised Epidemiology timespan and plotting

* configurable timespan can now state a fixed date in the past with a
  string <YYYYMMDD>.

* Epi plotting now generates two figures, one for integrated epi risk
  over past dates (from start of CalculationSpanDays until
   --start-date date), and a second for
  integrated epi risk over future dates (from date of --start-date
  until  end of configured CalculationSpanDays)