Fix wirelength_analyzer when terminating at a comb cell (e.g. LUT)

[eddieh-xlnx]

An example of this occurring is the case when the LUT drives a hierarchical (out of context) port, so it becomes the terminus.

[zakn-amd]

Lines 443 and 445 in the docstring should be updated to replace tuple with list.

Perhaps a warning should be emitted when terminating on a combinatorial cell, since this breaks the invariant "paths terminate on sequential cells". But, given the example above I suppose the invariant might not actually be invariant.