PERF: Reduce number of times findall is called on XML file

[greglucas]

The parameter types and parameters can be read through once and loaded into a cache straight away and then the lookups done on the cache after that. This prevents a findall() call iterating through the entire XML document every time a new parameter is found.

Profiling with the script provided in #112, we go from 12s to 0.03s for the Definition creation.

closes #112

Checklist

• Changes are fully implemented without dangling issues or TODO items

[greglucas]

Ping for a review here. I think this would really help IMAP as some of those XTCE files can be quite large and this should really speed up initial parsing without change in functionality.