preliminary implementation of ndigest
fix ndigest.neuronlistfh

* not allowed to set db@dir to NULL
proper documentation for ndigest methods
tests for ndigest
Merge branch 'feature/ndigest'

* feature/ndigest:
  tests for ndigest
  proper documentation for ndigest methods
  fix ndigest.neuronlistfh
  preliminary implementation of ndigest
Merge branch 'master' of https://github.com/jefferis/nat