Avoid unnecessary memory allocation.

[bangerth]

While looking around what is necessary for #13005/#12998/#12993, I also realized an inefficiency: In one place, we resize a vector, and then copy into that vector, but the copy operation releases the previously allocated memory and re-allocates it. We can avoid this.

/rebuild

[drwells]

Can we just use the default copy constructor?

[bangerth]

No, we can't, because we need to call Subscriptor(), not Subscriptor(src).

[masterleinad]

No, we can't, because we need to call Subscriptor(), not Subscriptor(src).

But they do exactly the same, see

dealii/include/deal.II/base/subscriptor.h

Lines 277 to 287 in 93102df

	inline Subscriptor::Subscriptor()
	: counter(0)
	, object_info(nullptr)
	{}
	inline Subscriptor::Subscriptor(const Subscriptor &)
	: counter(0)
	, object_info(nullptr)
	{}

.

[bangerth]

Hm. I had not realized this. I still like it being explicit -- we have plenty of other places where we are explicit about this as well:

> grep -r ': Subscriptor()' include/ source/
include/deal.II/lac/vector.templates.h:  : Subscriptor()
include/deal.II/lac/la_parallel_vector.templates.h:      : Subscriptor()
include/deal.II/lac/read_write_vector.h:    : Subscriptor()
include/deal.II/lac/read_write_vector.h:    : Subscriptor()
include/deal.II/lac/read_write_vector.h:    : Subscriptor()
include/deal.II/lac/read_write_vector.h:    : Subscriptor()
include/deal.II/lac/affine_constraints.h:  : Subscriptor()
include/deal.II/lac/trilinos_tpetra_vector.templates.h:      : Subscriptor()
include/deal.II/lac/trilinos_tpetra_vector.templates.h:      : Subscriptor()
include/deal.II/lac/trilinos_tpetra_vector.templates.h:      : Subscriptor()
include/deal.II/matrix_free/operators.h:    : Subscriptor()
include/deal.II/matrix_free/operators.h:    : Subscriptor()
include/deal.II/matrix_free/matrix_free.templates.h:  : Subscriptor()
include/deal.II/matrix_free/matrix_free.templates.h:  : Subscriptor()
include/deal.II/base/table.h:  : Subscriptor()
source/lac/trilinos_sparsity_pattern.cc:    : Subscriptor()
source/lac/trilinos_epetra_vector.cc:      : Subscriptor()
source/lac/trilinos_epetra_vector.cc:      : Subscriptor()
source/lac/trilinos_epetra_vector.cc:      : Subscriptor()
source/lac/trilinos_vector.cc:      : Subscriptor()
source/lac/dynamic_sparsity_pattern.cc:  : Subscriptor()
source/lac/chunk_sparsity_pattern.cc:  : Subscriptor()
source/lac/petsc_vector_base.cc:    : Subscriptor()
source/lac/petsc_vector_base.cc:    : Subscriptor()
source/lac/block_sparsity_pattern.cc:  : Subscriptor()
source/lac/trilinos_precondition.cc:    : Subscriptor()
source/base/quadrature.cc:  : Subscriptor()
source/base/quadrature.cc:  : Subscriptor()
source/base/quadrature.cc:  : Subscriptor()
source/base/quadrature.cc:  : Subscriptor()

[drwells]

Lets just merge this as-is - this PR is already an improvement. We can worry about consistently using default copy constructors later.