Added optional keys to circuit config

[joni-herttuainen]

Adding following keys:

• alternate_morphologies
  • a dictionary for different representations of the morphologies than the default .swc (e.g., .asc)
  • in components
• populations
  • can be used to override all or subset of the values in components
  • in nodes/edges
• type
  • can be used to define and access node/edge type for population (e.g., without having to open the nodes_file)
  • in populations