[Bug] Error thrown in Bragg2DPtycho if surface_normal and scattering_mode are None and no orientation specified in the dataset #65

The changes made in #63 will now cause the following error to get thrown if Bragg2DPtycho is used, but surface_normal and scattering_mode are both none and no orientation is specified in the dataset. The simplest way to reproduce the error is to run examples/fancy_ptycho.py but replace the cdtools.models.FancyPtycho.from_dataset with cdtools.models.Bragg2DPtycho.from_dataset.

Traceback (most recent call last):
  File ".../cdtools/examples/fancy_ptycho.py", line 8, in <module>
    model = cdtools.models.Bragg2DPtycho.from_dataset(
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../cdtools/src/cdtools/models/bragg_2d_ptycho.py", line 292, in from_dataset
    elif scattering_mode.strip().lower() in {'t', 'transmission'}:
         ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'strip'

The intended default behavior for the surface normal defining logic is to make surface_normal = np.array([0., 0., 1.]). The logic for defining the surface normal in Bragg2DPtycho needs to be rejiggered a bit to guard the string-stripping operation against cases where surface_normal and scattering_mode are both none.