Guidelines on discovering custom solvers

[MakisH]

If an application case requires a custom solver binary my-solver that is provided from another directory, the run.sh script should look for a my-solver binary in the PATH. It should not be required to manually copy (and rename) the file. Checking for both a binary in the PATH and alternatively a file in the application case directory would be fine.

This makes automation (e.g., testing) easier.

Examples:

• https://github.com/precice/tutorials/blob/develop/tools/run-dealii.sh
• https://github.com/precice/tutorials/blob/develop/perpendicular-flap/solid-dune/run.sh
• https://github.com/precice/tutorials/blob/develop/perpendicular-flap-stress/solid-gismo/run.sh