This is significant speedup for large simulations.

The element ids are used to check if globally the variable
has been deactivated for all surrounding elements making
application of dirichlet 0 bcs necessary to avoid singular
global system.

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

Defaulting ProcessVariable move ctor.

Adding a new convenience overload of getEssentialBCValuesLocal taking
whole mesh w/o nodes.

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.

Replacing the dummy segment with an optional one to avoid
floating point exception in the evaluation of the position
on the line. If the whole domain is deactivated, then there
is no need to evaluate a position on a line segment.

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.

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

Adding a single space in incorrectly broken lines.

Either time interval is given w/o line segment, which is
unused for whole material ids blocks deactivation *or*
a curve and line segment must be given.

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

Code simplification and one less dereferencing at run time.