[ogs] -m switch: fallback to reading gml from prj-file directory

When a mesh directory is given with the -m switch, the gml file is also read from the given mesh directory. This commit adds a fallback to also reading from the prj-file directory.

Motivation is when using partitioned meshes they often are created in subdirectories of the project. Before this one had to copy the gml file into that location too.

Also added an OGS_FATAL when gml-file is not found.

Also fixed a runtime warning when -o (output directory) is not given in ogs and partmesh.

1. Feature description was added to the changelog