Add default copy constructor for `RecombineVec`

Some compilers complain if the copy constructor is not explicitly
specified.
13 jobs for cast-call-for-component in 0 seconds
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed cdash-commit

passed ghostflow-branch-check-master

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:10:54

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:15:45

passed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:25:32

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:28:00

passed cdash-buildbot

passed cdash-buildbot

failed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:00:34

failed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:13:04

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:00:40

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:27:42