cylc-6 graphing bug fix and enhancements

The final cycle point for manual suite graphing was being computed automatically, if not given explicitly, as initial point plus twice the runahead limit.  This was broken by the new "N active cycle point" implementation of runahead limiting - on current master "cylc graph SUITE" will only graph a single cycle unless the suite uses the deprecated hard runahead limit.

This fixes the problem by computing a graph equivalent of the "active cycle points" and using it to define the final graph cycle point relative to the initial one.  N is configurable and defaults to 3, and can still be overridden by an explicit final point.  Works with suites containing different cycling intervals.

@benfitzpatrick - please review (as the active cycle point limiting was your invention).