PatternAttribution for iNNvestigate 2.0 version

The great method "PatternAttribution" was available in the previous iNNvestigate version (for tensorflow 1) and is not available anymore for the current version for tensorflow 2.

I wonder whether it is planned to implement the method again and if so when it will be available? :-)

In the `README.md` it is stated: _pattern.net: [PatternNet](https://arxiv.org/abs/1705.05598) estimates the input signal of the output neuron. (Note:_ ***not available in iNNvestigate 2.0***_)_