[CWEP002] Add support for computed attribute synchronization
Related to #3546717.
Test and handle the behaviour with several formulas and identified use cases. To
do so add a birth year and a computed attribute age to Person in the test
schema.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)