Add operation of LRP always applies the Z-Rule

Hello,

the LRP Add operation always applies the Z-Rule as stated in the documentation:
"""Special Add layer handler that applies the Z-Rule""" https://github.com/albermax/innvestigate/blob/master/innvestigate/analyzer/relevance_based/relevance_analyzer.py#L254

When using the `LRPAlpha1Beta0` analyzer, I would expect that the Z+ Rule is used, as the `a+b` can be seen as a linear layer: `[1 1] @ [a b]^t`. I think a similar problem exists for `AveragePoolingReverseLayer`.

Thanks,
Leon