bug fix: avoid descriptor exceptions on 0d arrays (e.g. strings)

[jooh]

check_descriptor_length casts each descriptor value to np.array. This can result in 0-d arrays if the input is str. The subsequent check on shape[0] then triggers an exception because 0-d arrays are weird and have no shape elements.

So I fixed that by flattening the array whenever the shape is empty. I also added a test.

Finally, I snuck in a stylistic change - single character variable names are rarely a good idea, and when the character clashes with a common pdb shortcut it's really ill advised.

[HeikoSchuett]

thanks for writing a test immediately. looks good and I am happy to remove the n

[JasperVanDenBosch]

+1