Add units to Orbital docs

[EmmanuelMess]

Add the units for everything under https://pyorbital.readthedocs.io/en/latest/#pyorbital.orbital.Orbital .

[mraspaud]

Thanks for the suggestion, PRs are welcome.

[gerritholl]

Thanks for the suggestion, PRs are welcome.

What are the units supposed to be? m or km?

Related discussion for satpy: pytroll/satpy#3182

[djhoese]

This was reported by another use in #203 with examples:

get_position is documented as returning "the cartesian position and velocity from the satellite". However, there are many different coordinate systems used for orbital mechanics, and a mix of different units used (metres and kilometres being most common).

For example, taking a TLE for the ISS, I get values such as:

position: [ 0.360051  , -0.99710063,  0.09493735]
velocity: [ 0.04398684,  0.01046155, -0.0561985 ]

Which don't match either the magnitude or direction of the ECEF position and velocity reported by other tools for the same TLE and time:

position (km): [ 6435.89390019 -2073.16708531   605.52322922]
velocity (km/s): [ 1.82908961  3.91010747 -5.97402701]

CC @DFEvans