Merge branch 'directedgraph'

* directedgraph: (31 commits)
  New function to make an igraph from a neuron type edgeless
  Missing doc
  typo - merge on rebase
  Fix remaining tests for Longair data by providing correct point sequence
  Fix one of the tests for Longair tracing data - now that ParseSWC gives sensible ordering
  defensively change degree -> igraph::degree
  Quick test for CoreNeuronFromGraph
  Add equivalence test with real neuron for two as.igraph methods
  Tests for new as.igraph functionality
  SWC2Neuron will now use a DFS normalised SegList starting from root
  ADD CoreNeuronFromSWC() to parse SWC data using igraph intermediate
  functional versions of as.igraph.(neuron|igraph|swc)
  methods to get root and branchpoints for neurons and (directed) igraphs
  More consistent set of (Double)EdgeListFrom(Neuron|SWC) functions
  SegListFromFullGraph -> CoreNeuronFromGraph
  Teach RerootNeuron to cope with directed graphs
  Teach SegListFromFullGraph to cope with directed graphs
  Fix 883e
  use degree rather than neighborhood.size
  Add proper tests for RerootNeuron and for new as.igraph.neuron
  ...