YT-CPPHL-11: Implement the bf-directed incidence-matrix model

[SpectraL519]

1. Created the impl::incidence_matrix class specialization for the bf-directed model, which:

• Stores a |V| x |E| incidence matrix, where each entry specifies the incidence type between a given vertex and hyperedge
• Defines the following operations:
  • Adding and removing vertices
  • Adding and removing hyperedges
  • Binding (tail/head) and unbinding vertices to hyperedges
  • Retrieving the degree, in-degree, out-degree of a vertex
  • Retrieving the set of hyperedges incident with, outgoing from, incoming to a given vertex,
  • Retrieving the number of vertices incident with a given hyperedge and the number of hyperedge's tail/head vertices
  • Retrieving the set of vertices incident with a given hyperedge and the hyperedge's tail/head vertices

2. Refactored the inner major/minor-specific logic in the existing undirected models to generically handle these operations
3. Renamed the n_vertices and n_hyperedges methods of the hypergraph class to order and size respectively to follow the proper terminology

[SpectraL519]

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