[feature][GridPtr] distinguish file format by extension.

6 jobs for feature/gridptr-can-read-gmsh in 10 minutes and 41 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #76509
duneci
debian:10 clang-7-libcpp-17

00:06:48

passed #76508
duneci
debian:10 gcc-8-noassert-17

00:10:41

passed #76511
duneci
debian:9 clang-3.8-14

00:05:07

passed #76510
duneci
debian:9 gcc-6-14

00:07:49

passed #76512
duneci
ubuntu:16.04 gcc-5-14

00:05:58

passed #76513
duneci
ubuntu:18.04 clang-6-17

00:07:18