Distributed graph loading

[maniospas]

Create a distributed implementation of graph loading, where each worker loads part of the same graph.

• To support interoperability between backends, create a class of Lazygraph that subclasses Fastgraph that replaces the whole initialization process within the preprocessor.
• This might require backend-agnostic preprocessing while disabling certain customiazations of the preprocessing. Getting to execute things in a distributed backend is the trade-of.
• Create a table of graph types with advantages and disadvantages in the docs.