More efficient querying of clock skew

[maliberty]

In OR when generating the metrics we do:

    report_clock_skew >> $filename
    report_clock_skew_metric >> $filename   // calls worst_clock_skew -setup
    report_clock_skew_metric -hold >> $filename  // calls worst_clock_skew -hold

once to get the text report, and separately to get the min/max skew values. Its quite expensive on a large design like megaboom to get this data and doing it three times is painful. We aren't even running report_clock_skew -hold which would make it four times.

I believe most of the runtime expense is not setup/hold specific but rather about tracing the reg2reg connections. Would it be possible to:

1. Have a single call compute both setup & hold in one go?
2. Have an API that can both issue the text report and return the worst skews so they can be added to the metrics?

[maliberty]

Relates to The-OpenROAD-Project/OpenROAD#6030