Added support for synapses

This is a big and unweildy update (the last one, we promise).
- added synapses
    - update to a version of modparser that generates net_receive and
      point process currents
    - added event_queue type for queueing events in a cell
    - added advance_to() method to FVM cell that supports event delivery
    - added synapse storage and modification to cell and fvm_cell types
    - added unit tests and NEURON validation test
- include support for const_views
    - moved to more recent bcumming/vector library with support for
      const_views
    - added const-safe const_view interace to fvm_cell and matrix types