1043 warm start follow up

[hjoliver]

This follows on from #1043, for #119. It [now!] just attempts to more clearly document the difference between cold and warm start, and the sole reason for using old-style cold-start tasks.

[hjoliver]

@benfitzpatrick - please review.

[hjoliver]

Ben - this is ready for review now. I temporarily forgot how my own b4stard child, the "cold-start task" worked - they do still need to be loaded as 'succeeded' in a warm start as you must have realised. So with this back on my mind I attempted to document more clearly the difference between cold start, warm start, and restart; and when (if ever) to use old-style cold-start tasks ... which had never really been done properly in the user guide.

[hjoliver]

(hang on ... maybe not ready - some tests are failing; groan).

[hjoliver]

I think it's ok, the failing tests are all the new graphing ones under tests/cyclers/ due to a default colour being lightgrey rather than black on my box.

[benfitzpatrick]

Looks good - I think we're getting closer to the big 6!