Use Process' implementation of active elements

The generalized implementation takes into account other process variables, not only the first of a process when computing the deactivated elements. This fixes a bug in domain deactivation when the first variables (like T) in a multi-variable process (like THM) was deactivated erroneously leading to deactivation of other variables (p and u).

An existing HT (where T is first and p the second variable) test for this feature is modified to see the influence of temperature deactivation on the pressure field.

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