#119: warm start support

This adds proper warm start support back for #119.

This is done by decoupling the "prerequisite-start" point ("start_point") from
the "sequence-start" point ("initial_point" or "initial cycle point").

"final_point" now means the "final cycle point", while "stop_point" is
a configured stop point e.g. via `cylc stop`.

@hjoliver, please review.