System and network class function members

[verri]

A proposal in this library is that all systems are Markovian with local dynamics. The function members of the classes that represent the networks and the systems should reflect such properties.

For the networks, some required members are:

• neighbors(index) const -> container<index>
• size() const -> size_t

For the systems, some required members are:

• iterate()
• time() const -> unsigned
• state() const -> container<vertex_state>