TMC: Enable Non sidereal tracking in for Low

Dependent on MCCS providing fucntionality.

Follows on from SP-4224 to complete the LOW portion of non-sidereal tracking:

What & Why?

Currently the TMC is only capable of controlling the MID and LOW telescopes to observe celestial objects which move in a sidereal manner (e.g. motion consistent with the rotation of the Earth). There are multiple uses for tracking objects which have non-sidereal motion (planets, satellites etc). 

The key feature of non-sidereal targets is that they are not at a fixed RA/Dec or Alt/Az, but instead have continually changing positions in both of those coordinate systems.

For many solar system bodies (Sun, Moon, planets and their satellites) there is support in katpoint's Target - this may be used to calculate positions at given times and to derive delays. For artificial satellites in earth orbit some are supported in katpoint (TBC) or a "Two Line Element" with orbital elements may be specified.

To track a non-sidereal target this may be used to provide the RA/Dec or Alt/Az input as a function of time.

For LOW the tracking position in RA/Dec sent to MCCS should be updated at required timestamps.

For MID the position in AltAz can be generated from Target for sending to dish.

Where does the target name or ephemeris table or tle table come from? For the targets supported by katpoint the OSO software will simply send the name of the required target. For less common targets the table will originate with the user and arrive via OSO. (This is lower priority at present).

The further step in this is that the celestial position used in the delay polynomial calculations also needs to be updated. Again common targets are in katpoint and that support can be used. At the moment it should be assumed that current polynomial update rates and validity periods may be used.

Who?

Science Operations.
Commissioning Science.
Astronomical Community.

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Scope of the feature:

• Define the OSO-TMC schema for low. (In case of Mid, the “reference_frame" and "target_name" are provided under the ‘pointing’ section. Similar details will be required under the MCCS section for low (sky_coordinates) ( current agreement : https://app.slack.com/client/T1T3Q6ZR9/CDFEJ5FH7)
• Update the schema in the SKA Telescope Model, and validate it using TelModel or CDM
• Update TMC low SubarrayNode to use the OSO-TMC schema for non-sidereal tracking and utilize the CDM/Ska Telescope Model for schema validation
• Modify the Mccs Subarray leaf node, utilize KATPOINT Api
• Low integration with new BDD test case