Implement DVLA CEG excel parser
wip
Refactor splinter bucket contents steps

Have them both talk in terms of buckets; it's part of our ubiquitous language and the sentence reads better.
Match the containment statment to hamcrest as we're using that in a lot of places already so it seems to make sense to use their language.
Factor upload parsing out of flask code
Rename parsing package to upload

To avoid confusion with query parsing and record value parsing.