Feature request - support .vcd files that do not start at cycle 0

[oharboe]

If a .vcd file does not start at cycle 0, then the clock period is inferred incorrectly from the .vcd and it produces a warning like:

Warning: clock clock vcd period 7256333312.000 differs from SDC clock period ...

Normally a .vcd file starts on cycle 0:

$enddefinitions $end
$comment Show the parameter values. $end
$dumpall
r2.5 &
r5 %
$end
#0
$dumpvars

However, if a .vcd file is a snippet from a longer run, then it could start at cycle non-zero, like, 7000 below:

$enddefinitions $end
$comment Show the parameter values. $end
$dumpall
r2.5 &
r5 %
$end
#7000
$dumpvars