Thread `Atomic` side-condition through `ElimModal`, `ElimAcc` and `ElimInv`.

This makes sure that when trying to open an invariant or to eliminate a mask-changing
update around a non-atomic WP that it doesn't fail with "cannot eliminate modality",
but instead gives an side-condition `Atomic ...` informing the user what's going on.

Unlike the class `ElimModal` and `ElimInv`, the class `ElimAcc` was not yet equipped
with a Coq side-condition. This commit adds such a side-condition.