(solarsystem) use km as length unit for solar system setup

[danieljprice]

Type of PR:
Bug fix

Description:

• Using au as the units for small bodies in the solar system causes problems with finite precision (round off error). This is now changed to km as the length unit
• fix issue with datetime string for epoch being converted automatically
• better error handling if ephemeris files cannot be downloaded

Testing:

• checked that orbits of the 9 planets+moon still works in these units

Did you run the bots? no

Did you update relevant documentation in the docs directory? no