Add LRP support for nested Chains

[adrhill]

This will be the first step towards #10 by allowing nested model structures.

This requires the following changes:

• [ ] new internal representation of rules, e.g. via LRPRulesChain / LRPRulesParallel to support graphs
• [ ] treat chains the same way as layers by adding lrp!(Rₖ, r::AbstractLRPRule, c::Chain, aₖ, Rₖ₊₁), which can be called recursively. This will require a different approach to pre-allocating activations and relevances than the one currently used in the call to the analyzer.
• [ ] Composite might require a refactoring
• [ ] Update lrp/show.jl