graph: add --flatten-icp option

[oliver-sanders]

Add a cylc graph --flatten-icp option for "snipping" edges from R1 cycles for graph simplification.

This is the trivial idea mentioned in previous discussions around isolated startup graphs.

This preserves the ICP dependence (i.e, there's no information missing from the graph), it just duplicates the R1 dependent tasks into their respective cycles (but outside of the dotted cycle boxes which appear with the -c option) to flatten out these inter-cycles.

Without this option (present behaviour / default):

With this option:

The R1 label is completely optional, have done it this way for clarity, but could replace with the actual cycle point, or template, whatever. Just let me know what the preference is.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

@hjoliver, I have closed the side PR for now.

Pending the outcome of other conversations, we can open this against cylc-flow@master as required, but this option is orthogonal to that one and will still standalone and provide benefit without it - so no need to couple the two and delay this one. I have cherry-picked your fix.