Implement strict typing in existing code

And the failed test shows exactly why it is so important to use strict typing (and why it is important to have working tests like we now do!) - somewhere in the code we have "energy" being passed around as a string instead of a float. That error was being hidden due to PHP type coercion taking place when strict typing was disabled.

Mmm ... cascading failures. I might have to back off and only use strict types for part of the code for the time being, because fixing all the tests might prove too time consuming for now.