Make it possible to load SPICE kernels individually and at runtime

[JonasGilg]

SPICE does support loading kernels individually. It would be really great if we could simply point to a single dataset and load it either at the start of CosmoScout or even at runtime.

When we can load them at runtime we could maybe even hook into the SPICE databases from NASA and ESA and load missions on demand via the UI.