Generating list of don't cares

[wagenaartje]

Hi, i'm not sure if this is already possible with the library, but it would be nice to automatically generate a list of "don't cares". For example, if I have string-encoded inputs

ones = ['010', '001']
zeros = ['110', '101']

And when I simplify the expressions I get

qm.simplify_los(ones) # {'001', '010'}

However, I desire output {'0--'} based on the above data. At this moment I can achieve this by generating all states that are not inside ones or zeros, but it would be nice to have this functionality built in to the library. For example as follows

dc = qm.generate_dc_los(ones, zeros) # 
qm.simplify_los(ones, dc) # {'0--'}

Let me know if more people desire this, then I will create a PR :)