LRP

* Restructure code.
* Add pooling layers. (max pooling is covered with the gradient, sum pooling should work with LRP rules; means need to add reverse hook).
* Add merge layers; i.e. add reverse hooks.
* (still needed?) Add LNR layer to innvestigate.layers and create a LRP rule to handle it.
* Add check for all keras layer if compatible, if not raise exception.
* Get to work for all application networks.
* Organize module: which rules to keep, divide DT and LRP?