fix: actually use default non_matching_subset
new class scoringmatrix (inherits from table)
give neuron_pairs query and target arguments

* probably won't be much used, but does allow it to construct pairs of neurons with
  different query/target sets with ease.
Fix bug in neuron_pairs when query!=target

* also change to ignoreSelf arg for consistency
refactor: expand.grid call -> neuron_pairs
Add some random neurons as test data
Fully worked example
Depend on nat >=1.5.12 for c.neuronlist

* this is just for the example which combines neuronlists with incompatible
  attached data.frames
Merge branch 'feature/make-smat'

* fully functional scoringmatrix code
* could still do some more work on user interface/function names
* feature/make-smat:
  Depend on nat >=1.5.12 for c.neuronlist
  Fully worked example
  Add some random neurons as test data
  refactor: expand.grid call -> neuron_pairs
  Fix bug in neuron_pairs when query!=target
  give neuron_pairs query and target arguments
  new class scoringmatrix (inherits from table)
  fix: actually use default non_matching_subset