#119: warm start support

[benfitzpatrick]

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.

[hjoliver]

@benfitzpatrick - looks good, but:

Your third commit restored the bin/cylc-run:load_tasks_warm() method, but I think we can get rid of that now, and replace it with simply ignoring declared cold-start tasks at warm start-up (even that's only needed for back-compat because new cold-start tasks should be in an R1 which will be out of bounds in a warm start, right?). Loading declared cold-start tasks in the succeeded state is definitely no longer necessary now that pre start point triggers are ignored. The only other difference between the current cold and warm load_tasks methods is use of the startup flag passed to task proxy class init, and in fact that should be True for warm and cold start - the flag is only used to trigger adjustment of the suite start point up to the next valid point for the task, and that should happen for warm start too. Currently it doesn't: e.g. for [[[T00]]] foo if you warmstart at 12Z the first instance of foo will be - wrongly - at 12Z. Then, while you're at it you might as well get rid of the old raw start method.

[hjoliver]

Ben - your branch needs an update. My suggested changes, if you agree, should be pretty trivial ... then we might be able to merge the #119 feature branch to master at last 😃