[SPIIP-77] Treat 'negative_latency' as zero if not specified.

Closed Patrick Clearwater requested to merge pwc_neglat_default into master

This allows the up-to-date version of the code to read older bank files.

The code before the change is (roughly):

for root in (list of possible roots):
    negative_latency = get_from(root)

which means that we always get the last negative latency from that list... not sure if this behaviour is documented anywhere (or whether it applies), but I’ve preserved that.

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