Enable -Werror with CMake flag (on by default).

Added additional information on fixing warnings to CONTRIBUTING.md.
Status Job ID Name Coverage
  Docker Cleanup Image
skipped #11980
docker-cleanup-nektar
 
  External
passed CentOS 7 64-bit (default)

00:19:28

passed CentOS 7 64-bit (full)

01:09:45

passed Debian 10.0 64-bit (default)

00:09:08

passed Debian 10.0 64-bit (doxygen)

00:04:44

passed Debian 10.0 64-bit (fulltut)

01:32:21

passed Debian 10.0 64-bit (tutorials)

00:04:16

passed Debian 10.0 64-bit (userguide)

00:06:44

passed Debian 8.0 64-bit (full)

01:02:03

passed Debian 9.0 32-bit (default)

00:13:40

passed Debian 9.0 32-bit (full)

00:54:35

passed Debian 9.0 64-bit (default)

00:10:47

passed Debian 9.0 64-bit (full)

00:45:56

passed Fedora 25 64-bit (default)

00:12:55

passed Fedora 25 64-bit (full)

00:55:16

passed Ubuntu 14.04 64-bit (full)

01:51:35

passed Ubuntu 16.04 64-bit (default)

00:11:43

passed Ubuntu 16.04 64-bit (full)

00:40:45

passed Ubuntu 18.04 32-bit (default)

00:12:54

passed Ubuntu 18.04 64-bit (default)

00:12:08

passed Ubuntu 18.04 64-bit (full)

00:46:43

passed Windows 7 64-bit (default)

01:44:03