Introduce Filter model and import them from JMP files
Allow import of formula-based answers

Some questionnaires (SVRS98) use simple formula for answers. We now allow
them to be imported as hardcoded values (formula is lost). Also we do not
import null values anymore.
First working computation
First working chart
10x faster computing with in-memory caching (and fix tests)

Profiling with xdebug and kcachegrind allowed us to drammatically
improve performance