Typo in class name: AdjustableEffectivePermittivityMixin missing an "s"

[zz246]

In smrt/emmodel/common.py, the class is currently defined as:
class AdjustableEffectivePermittivityMixin(object):
However, other parts of the code (e.g., in symsce_torquato21.py) attempt to import it using the name AdjustableEffectivePermittivityMixins (with an "s"), which leads to an ImportError.

[ghislainp]

It is a bug indeed. Thank you for reporting. It is solved in the last commit.