Simplify the system that reactively triggers realizations, at the cost
of removing the preconditioning step described in issue #101 and doing
unnecessary realizations after certain kinds of updates.
The new system should trigger a realization after any update that could
affect the dimension of the assembly's deformation space. For
simplicity, any update to the regulator list triggers an update, even if
it doesn't affect the set of constraints. In particular, adding a
regulator triggers an unnecessary realization.
