Add some code quality checker / code metrics

[octopusinvitro]

I think our classes are pretty straight-forward and I've been trying to escape conditionals with duck typing etc. etc. but we might get a surprise here.

There are many static analysers out there but maybe something like CodeClimate or something that gives us a flog score.

[mhl]

RuboCop does some useful code complexity checks already, since I had to simplify the hacky bin/build_summaries.rb script to get them to pass :) e.g. it includes:

• Metrics/CyclomaticComplexity
• Metrics/PerceivedComplexity
• Metrics/AbcSize

It might be interesting to have addition checks, though.

[mhl]

Timebox to 1 day