Systematize the sub-ecosytem feature

Right now, MultilayerGraphs.jl should allow the implementation of custom multilayer graph types, making it a sub-ecosystem of Graphs.jl, specialized in multilayer graphs. The way NodeAlignedEdgeColored(Di)Graph has been implemented should prove it.

Anyway, it is not yet clear which methods should be implemented by the developer who wishes to extend the sub-ecosystem. Moreover, the current code has been written with the intent to repeat itself as rarely as possible, which may get in the way of such an effort.

We should then:

1. Agree on a set of APIs the external contributor should implement, and allow this to happen by restructuring the code;
2. Systematize the matter in a dedicated section of the documentation (maybe along with the developer guide?).