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