Removing unique_ptr and constness from the members allowing removing the
constructors.

Defaulting ProcessVariable move ctor.

The previous method was based on material ids, but now
it is gone there must be another method to find out
whether an element is active or not.

A deactivated subdomain consists of a single set of elements
(with inner and outer nodes) instead of multiple meshes, one
for each material id.

In the former implementation the artificially created
boundary between two deactivated subdomains was not handled
correctly resulting in singular global matrices.

Some parts of the code expect an empty set of active elements
if all of the elements are active.

The previous implementation was looking for one deactivated
subdomain, but there could be multiple subdomains inactive
at the same time. Now all deactivated subdomains are
searched for active/inactive elements.

Allowing not only a zero value to be applied to the inactive
nodes but also heterogeneous, time-dependent values.

Using elements centers for the condition.

Creating the curve for time interval s.t. the whole material ids group
is deactivated, as it was for the time interval case.

The set of deactivated nodes is now constructed on the fly using the
active elements set passed from the process variable.

Code simplification and one less dereferencing at run time.

Same thing is checked after the find_if.

The name reflects the special nature of the nodes,
namely that these are nodes not touching the active
elements.