Feature/invariant refactoring

For invariants: moved output to always be the first argument (consistent with violations for constraints)
Made all invariant/constraint/view constructors explicit
Made unary constraints/invariants (ElementConst, InDomain, InSparseDomain) into views
Renamed some views by replacing the View suffix with the Const suffix
ElementVar and ElementConst now accepts an optional offset argument in their constructor

astra-uu-se / atlantis