Make the return type of `biolqm.convert_trace` consistent

[jonrkarr]

biolqm.convert_trace returns empty lists or non-empty iterables. It would be helpful to consistently return one type of data structure.

[aurelien-naldi]

Looking at the code, I'm not sure why this was implemented like this. It returns an empty list only when it is called without anything to work with. We could maybe make it return a fake iterator instead, but it could just assume that it has a valid parameter!

[jonrkarr]

Raising an error would be even better.