Make numpy-quaternion an optional dependency

[hakonanes]

I see numpy-quaternion has become a NumPy-2.0-transition-bottleneck downstream for pyxem (pyxem/pyxem#1099). There's discussion of our use of this package in #506 and #507.

I suggest to make numpy-quaternion an optional dependency.

We use numpy-quaternion for three operations:

• Conjugation:

  orix/orix/quaternion/quaternion.py

  Lines 208 to 214 in 688ee4a

  	@property
  	def conj(self) -> Quaternion:
  	r"""Return the conjugate of the quaternion
  	:math:`Q^* = a - bi - cj - dk`.
  	"""
  	Q = quaternion.from_float_array(self.data).conj()
  	return self.__class__(quaternion.as_float_array(Q))

• quaternion-quaternion multiplication:

  orix/orix/quaternion/quaternion.py

  Lines 222 to 225 in 688ee4a

  	if isinstance(other, Quaternion):
  	Q1 = quaternion.from_float_array(self.data)
  	Q2 = quaternion.from_float_array(other.data)
  	return other.__class__(quaternion.as_float_array(Q1 * Q2))

• quaternion-vector multiplication:

  orix/orix/quaternion/quaternion.py

  Lines 226 to 233 in 688ee4a

  	elif isinstance(other, Vector3d):
  	# check broadcast shape is correct before calculation, as
  	# quaternion.rotat_vectors will perform outer product
  	# this keeps current __mul__ broadcast behaviour
  	Q1 = quaternion.from_float_array(self.data)
  	v = quaternion.as_vector_part(
  	(Q1 * quaternion.from_vector_part(other.data)) * ~Q1
  	)

All places for speed, not for functionality. We had implementations of these things without numpy-quaternion before. We can re-introduce these computations as a fall-back if numpy-quaternion is unavailable.

As an added bonus to this work, I want to make it easier to make dependencies in orix optional. numpy-quaternion is a good place to start.

We can do this fairly quickly. If interest, we can make a v0.13.1 patch release.

[hakonanes]

I've gotten one 👍🏻, so I'm going for it

[CSSFrancis]

@hakonanes That would be fantastic! I think @ericpre would also be very happy :)

[pc494]

I certainly will be very happy. I assume the speed trade-off is significant, but the documentation can be marked up to reflect that.

[CSSFrancis]

I certainly will be very happy. I assume the speed trade-off is significant, but the documentation can be marked up to reflect that.

I think it's a factor of 2 is what @hakonanes said. I feel that is pretty minimal, there are other places in orix where it would be easier to speed things up if we really needed it.