spelling inconsistency

[bruceable]

Not super important but more of a nuisance to readers:

Word Occurrences
inferable 16
inferrable 4 (github says is incorrect)
uninferable 12
uninferrable 1

[IainCRobertson]

Thanks Bruce. Inferrable is indeed what we should use, and what appears in the Siemens docs. These errors were mostly imported from the original E-Trace spec, and I hadn't realized quite how inconsistent that was! All fixed now - see #28