Add dither cmd sets and fix nsm cmd_set, etc

[taldcroft]

Description

This adds dith_on and dith_off command sets. It also updates the nsm command set to include scs107 and dith_off.

Testing

• Passes unit tests on MacOS (new tests)
• [n/a] Functional testing

Fixes #50

[jeanconn]

I thought the fundamental issue was that we still want to modernize to Ska3, update testing, and avoid backports. Hence #51 has been hanging around.

[taldcroft]

Argh. Forgot there is this whole mess of Ska2 code still going.

[taldcroft]

From Gregg, so we probably want to update the scs107 state.

Recently, SCS-106 was updated to leave 3 FEPs on rather than no FEPs on.
SCS-106 is run as part of the SCS-107 sequence.

Therefore thermal model history assembly requires that this change be reflected
in any shutdown: both science-only shutdowns as well as Full Stops.  Also, the
ACIS Team recently added new power commands which have to be handled by Backstop History.

[taldcroft]

Should we just merge this?

[jeanconn]

Yes. Though not sure if it is worth cutting the ska2-compatible bits.

[taldcroft]

The Ska2 compatibility is not compromising the Ska3 testing, so it can stay.