Handle edge cases from add()ing elements twice

[anderoonies]

Resolves #120

Summary

This fixes a unique edge case that could pop up if an element was add()ed twice. Adding an element only expands elementPoolSize by one, but after remove()ing that element, subsequent add()s would cause all duplicates to become active at once.

Changes:

• Mark elements as _sortInvalidated when they're remove()d so that any subsequent add()s re-trigger a sort
• When sorting elements in redraw, rather than counting the number of elements removed to shrink elementPoolSize, recalculate elementPoolSize by finding the last non-.alive element. This fixes the case where multiple elements are added back with a single add and elementPoolSize needs to be recalculated.

Checklist

• npm test passes
• Unit tests are included / updated
• Documentation has been updated where relevant