Add new ephemeris data source / offline mode

[jgieseler]

Right now, all ephemeris data is obtained dynamically from https://ssd.jpl.nasa.gov/horizons. For times when this service is not reachable - or as an offline version - some alternative would be nice.

1. https://omniweb.gsfc.nasa.gov/coho/helios/heli.html takes the data from there (but doesn't seem to have an API)
2. More coordinates, and some planet-centered coordinates, are found in spacecraft-specific directories at https://spdf.gsfc.nasa.gov/pub/data/, for example https://spdf.gsfc.nasa.gov/pub/data/stereo/ahead/helio1hr/

The data from https://spdf.gsfc.nasa.gov/pub/data could be downloaded (for commonly used objects) and stored locally, so that offline-use would be possible.