Merge branch 'version_0.6.1' into 'master'

On() and Off() to base device, plus event pushing

This merge request does the following:
* Adds support for the ON state in the SKABaseDevice. This seems to be a minimum requirement for devices that subclass SKABaseDevice to be functional/extensible.
* Moves the commands On() and Off() from the SKASubarray to the SKABaseDevice.
* Adds pushing of change and archive events for changes to device State, adminMode and obsState
* Adds some test infrastructure for testing events
* Remove all mention of polling in XMI files, and disable all attribute polling in code (only obsState and obsMode affected).

These changes are non-breaking, so the version is bumped to 0.6.1.

See merge request ska-telescope/lmc-base-classes!23