Git Submodules

We are using more and more Git submodules in the ResInsight repo. Current submodules are Qwt, GSL and vcpkg.

When the content of the external repo is updated, we need to update the committ of the external repo we want to use. This requires more git commands, and can cause confusion for the developer.

Main references

Description of use of sub modules

Other References

https://leimao.github.io/blog/Git-Submodule/