consistency with additional and job_options

[soxofaan]

A bit related to #681:

We have a bit of inconsistent API regarding "additional" properties and "job options":

• Connection.create_job() only has argument additional. And, confusingly, puts this under an extra "job_options" field in the request
• DataCube.create_job() (and alike) only have argument job_options, which is passed to additional in Connection.create_job
• Connection.download()/DataCube.dowload() (and alike) have neither (support "job options" for sync processing #681)

I think it's best to align everything.
We can not really remove one or the other: additional is more in line with official spec, so should be kept/added where necessary. job_options is less official, but heavily used in existing code, so we can not just remove it.
Best seems to just support both additional and job_options

[soxofaan]

done with ec66b31 from #684