Default kernels location for the spacecraft with a space in their name

The user kernels directory can be provided on a spacecraft by spacecraft basis with an environment variable: $KERNELS_JUICE.

Unfortunately, some spacecraft have space in their name: EUROPA CLIPPER and the tool search for KERNELS_EUROPA CLIPPER instead of KERNELS_EUROPA_CLIPPER.

This could be fixed by replacing the SPACE by a _ before searching for the environment variable.

Temporary workaround:

Use an explicit kernels_dir='...' in TourConfig.