set solution vector to zeros before solve

4 jobs for feature/operators_in_dofmatrix in 18 minutes and 36 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #6702
debian:10 clang-6-libcpp-17

00:18:36

passed #6701
debian:10 gcc-8-17

00:12:48

passed #6703
debian:9 gcc-6-14

00:11:27

passed #6704
ubuntu:18.04 clang-6-17

00:18:27