The `Beam` model should not allow non-orthogonal polarization normal

[dagewa]

I have a beam model:

In [9]: print(self.experiment.beam)
Beam:
    wavelength: 0.0250824
    sample to source direction : {-0,0.0269665,0.999636}
    divergence: 0
    sigma divergence: 0
    polarization normal: {0,1,0}
    polarization fraction: 0.5

that will not rotate:

In [10]: self.experiment.beam.rotate_around_origin(axis, angle, deg=False)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
~/sw/cctbx/modules/dials/util/export_pets.py in <module>
----> 1 self.experiment.beam.rotate_around_origin(axis, angle, deg=False)

RuntimeError: dxtbx Internal Error: /home/fcx32934/sw/cctbx/modules/dxtbx/src/dxtbx/model/beam.h(436): DXTBX_ASSERT(std::abs(direction_ * polarization_normal_) < EPS) failure.

[dagewa]

I'm not sure what the point of the assert is here. It seems to be checking that the polarisation normal and the direction is orthogonal, but why do that here and not when creating the beam. Also, this is an unpolarised beam model.

[dagewa]

Further to this, when dials.refine changes a beam model it does so through the set_s0 method. This does not update the polarisation vector, which is probably how we can get into the above situation in the first place.

[dagewa]

Keeping this open, because there is a dxtbx issue in that using set_s0 allows one to set an inconsistent beam model, by not adjusting the polarisation normal vector.

[dagewa]

In general, the Beam constructors allow setting the s0 vector arbitrarily, while setting the polarisation normal along +Y https://github.com/cctbx/dxtbx/blob/20d4cf201afe4465e0d01bbfb50dfb688b33b463/src/dxtbx/model/beam.h#L115-L125 This allows for the construction of physically inconsistent models.

[dagewa]

Note this problem affects files used in tests. Loading experiments from $(dials.data get -q l_cysteine_4_sweeps_scaled)/scaled_35.expt, then the beam cannot be rotated:

>>> beam.rotate_around_origin((1,0,0), 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dxtbx Internal Error: /home/fcx32934/sw/cctbx/modules/dxtbx/src/dxtbx/model/beam.h(373): DXTBX_ASSERT(std::abs(direction_ * polarization_normal_) < EPS) failure.

this is because that file dates from before when #1939 was merged.