Add Edges interface

• Add a Edges interface to query edges regardless of the population names (#112)
• Add a CircuitEdgeIds object to contain the edge circuit ids (#112)
• Add a ids function to the EdgePopulation class to keep the Edge/Node class homogeneous (#112, #115)
• Add a get function to replace the properties function to the EdgePopulation class to keep the Edge/Node class homogeneous (#113)
• Add a network.py module with a NetworkObject abstract class to factorize the Nodes and Edges classes (#113, #114)
• Deprecation of the properties function from the EdgePopulation (#113)
• Add a _doctool.py module with a DocSubstitutionMeta class to update inherited class docstrings (#113)