Make rotate attribute of type teidata.numeric

[rettinghaus]

This PR changes zone@rotate into teidata.numeric in order to allow negative and non-integer values.
Closes #2637

[ebeshero]

@rettinghaus Thanks for this! Council members raise a good question of why we might have limited ourselves to positive integers only in the first place, so we want to discuss this first before deciding whether to merge!

[martindholmes]

There isn't a single example of the use of @rotate in the Guidelines, nor is it mentioned anywhere in the prose. That's no reason to reject this PR, but it would be good to see some examples. This might be an easy one to start with:

https://graves.uvic.ca/gr01_0472_004.html