Rename get_array_range -> get_z_range

[graeme-winter]

This sounds like a floating point / real valued index

What is z for a stack of stills?

What is z?

Proposed by @dagewa in

#186 (comment)

[graeme-winter]

I for one do not understand why I would want to get a real valued array range if my array bounds are literally integers (natural numbers, strictly)

There is no way I can understand it being useful to report the array range or similar as floats. Yes, you can have a floating point position within the array, in which case the caller is at liberty to cast.

N.B. get_angle_from_z or similar seems legitimate