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 Name Job ID Coverage
  Test
passed debian:10 clang-6-libcpp-17 #6702

00:18:36

passed debian:10 gcc-8-17 #6701

00:12:48

passed debian:9 gcc-6-14 #6703

00:11:27

passed ubuntu:18.04 clang-6-17 #6704

00:18:27