Fix an issue for datasets with colinear translations

[allevitan]

There was an issue in the calculation of the scaling for the scaled mean diffraction pattern, which would cause a crash whenever dataset.inspect() was called on a dataset with fully colinear scan points. A special case of this which happens a lot is when there is no scanning, or all the scan points are equal in the dataset.

I updated the functions to revert to just displaying the mean pattern in this case, and raising a warning. I also updated the docstrings. I did not add any tests because coverage for the dataset classes is quite sparse and feels like a separate project.

To test this specifically, one can run a modified version of inspect_dataset.py from the examples:

import cdtools
from matplotlib import pyplot as plt

# First, we load an example dataset from a .cxi file
filename = 'example_data/AuBalls_700ms_30nmStep_3_6SS_filter.cxi'
dataset = cdtools.datasets.Ptycho2DDataset.from_cxi(filename)

dataset.translations[:] = 0

# And we take a look at the data
dataset.inspect()
plt.show()

[yoshikisd]

I've tried the example you provided on my machine and it seems to work as intended.

These changes look good to me.