Ska api fixes

[javierggt]

Description

This makes the following changes to the ska_api blueprint:

• return error codes when there is an error
• add a few more functions I want to use to the API:
  • agasc: get_star, get_stars, get_agasc_cone
  • mica.archive.aca_dark.dark_cal: get_dark_cal_ids, get_dark_cal_id, get_dark_cal_image, get_dark_cal_props
  • kadi.commands: get_observations, get_starcats
• numpy arrays output from ska_api functions are now encoded
• added ska_api unit tests

Interface impacts

• Adds nine functions to the web API
• The API can now return 500 error if the output cannot be encoded, unless the argument strict_encoding=false is passed

Testing

For testing, this requires sot/kadi/pull/241 and sot/mica/pull/273.

Astromon tests fail, but this is independent of this PR. For astromon tests to pass one needs sot/astromon/pull/26 and some test data.

Unit tests

• On OS-X

Functional tests