Verbatim copy of nat functions for direct potential synapses
Add functions for approx potential synapses
new interface function potential_synapses
process neuronlists with same directionality

* the direct method is presently asymmetric
tidy up and document
Add missing function when using bounds
Add doi url for Stepanyants ref

* can’t seem to get \href to behave though
Make sure that we return 0 when inputs are missing rows
formatting
Merge branch 'feature/potential_synapses'

* first functional version using cleaned up version of Cell 2007
  paper implementation.
* clearly lots of potential for optimisation in direct method
* feature/potential_synapses:
  formatting
  Make sure that we return 0 when inputs are missing rows
  Add doi url for Stepanyants ref
  Add missing function when using bounds
  tidy up and document
  process neuronlists with same directionality
  new interface function potential_synapses
  Add functions for approx potential synapses
  Verbatim copy of nat functions for direct potential synapses