ndigest.neuronlistfh includes key fields only

* only keyfilemap and df
* everything else is redundant or irrelevant
fix bug in locating cmtk from wrapper script

* closes #156
Upgrade to roxygen2 4.0.2

* nb this changes the order of a number of arguments in man files
  (for the better)
Add bug reports URL and edit description.
Bump version to 1.5.13
switch from RANN::nn2 -> nabor::knn
Update NEWS to reflect RANN->nabor switch
Fix minor typo in README