enrollcast 0.2.0

• swing_schedule() now moves classes up and adds a new entry class every
  year, including swing and recovery years (#8). Swing years are rescaled to
  the base total and recovery years to the totals you supply, and the rescaled
  grades are the ones that move up the next year. After recovery nothing is
  rescaled: the projection continues from the recovered grades with the
  supplied entry values.
• Breaking: swing_schedule()'s recovery now takes annual school totals
  instead of multipliers, grade-specific recovery matrices are no longer
  accepted, and entry must cover the whole horizon. The vignette shows how to
  convert older calls.
• swing_schedule()'s ratios can be a list with one ratios table or
  projection matrix per year, for ratios that change or grades that open later.
• Schedule steps accept an optional total that rescales that year's grades;
  "base" means the base-year total. The rescaled grades carry into the next
  step.