Do not error on CIF files with multiple uisos at given position.

[tkittel]

As reported during the Neuwave NCrystal workshop, it is possible to encounter CIF files with multiple uiso values for a particular spot (e.g. on position 0.5 0.5 0.5 one might find 40% Al with uiso=0.01 and 60% Cr with uiso=0.02). Currently this triggers an error (while we accept a mixed element if the two uiso values are the same.

We could instead simply average the uiso numbers and issue a warning.