Add default assignment to sort_sublattice_intensities

[michaelhennessyjr]

If _sort_sublatticeintensities fails to assign an elemental configuration to a given site, it either assigns the highest intensity configuration or the most common sublattice element (found using _get_most_common_sublatticeelement). This works well, except for when the most common sublattice element is blank (''), as the function will find all the blanks and replace them with blanks.

It shouldn't be the case that the most common sublattice element is blank, but it might be useful to include a parameter in the function for a standard element to use as a default assignment for cases like this.

[PinkShnack]

Hey Mike, did you ever implement this? :)