YAN-1351: add tmc leaf node

This changeset adds a mock device for ska-tmc-dishleafnode to be used in SDP integration testing.

The tango interface for this device is similar to dish-master by using an epoch offset in seconds, however as it stands this appears to use datetime.timestamp which is an offset from the UNIX epoch (which again is different from the MJD and 2000 TAI epochs).

ECP-230031, ADR-39 and ADR-78 if I understand correctly all suggest that there shall only be two formats used for times, UTC (ISO-8601) for structured timestamps and Y2000 TAI offset times, thus the second format should be used everywhere eventually.