Off by one error in `Maestro._get_group_modes()`

[abraha2d]

https://github.com/alexdutton/python-maestro-servo/blob/main/maestro/device.py#L81 is currently:

for i in range(self.channel_count // 4)

This does not work with the Micro Maestro (and probably also the 18-channel Mini Maestro).
The list ends up with 1 group mode (corresponding to only 4 channels) instead of the required 2 (to support all 6 channels).

It needs ceiling division, not floor division:

for i in range(math.ceil(self.channel_count / 4))

Or, my personal favorite: upside-down floor division.

for i in range(-(-self.channel_count // 4))

[abraha2d]

Will try to submit a PR soon