removed some warnings

4 jobs for feature/interpolation in 3 minutes and 57 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #7479
debian:10 clang-6-libcpp-17

00:03:40

passed #7478
debian:10 gcc-8-17

00:03:57

passed #7480
debian:9 gcc-6-14

00:02:35

passed #7481
ubuntu:18.04 clang-6-17

00:03:13