Merge branch 'patch-ide_files' into 'develop'

Patch ide files

This patch excludes existing IDE-specific files from the git index. I'm not sure yet if this solution works clean for everybody, so please test it first locally.

@wyleung & @p.j.van_t_hof , this is what we discussed earlier with the `*.iml` files and `.idea/*` files.

See merge request !98