new OP `HZ`, converts v/oct to hz/v

[a773music]

What does this PR do?

Add a new OP HZ for converting v/oct to hz/v

Provide links to any related discussion on lines.

https://llllllll.co/t/teletype-3-2-feature-requests-and-discussions/32052/369

How should this be manually tested?

HZ V -2 = VV 25 HZ V -1 = VV 50 HZ V 0 = V 1 HZ V 1 = V 2 HZ V 2 = V 4 HZ V 3 = V 8

Any background context you want to provide?

Allows pitch control from the teletype to synths with hz/v tracking like the Korg MS-20

If the related Github issues aren't referenced in your commits, please link to them here.

I have,

• [x] updated CHANGELOG.md
• [x] updated the documentation
• [x] updated help_mode.c (if applicable)
• [x] run make format on each commit
• [x] run tests

[a773music]

Any thoughts on this? Things I should do differently or improve?

[tehn]

looks good to me! @scanner-darkly feel free to merge after you get a quick glance