Give nlapply a subset argument

* and use it in xform and mirror.neuronlist
* also includes a test
doc: mirror(.neuronlist) doc fixes
Give mirror.neuronlist OmitFailures arg

* this and subset will be passed on to nlapply
docs: small fixes to dotprops docs
add example listing all neuronlist methods
missing docs for as.neuronlist.default
more neuronlist doc fixes/additions
give dotprops.neuronlist OmitFailures argument

* will be handled by nlapply
Merge branch 'feature/nlapply-failures'

* feature/nlapply-failures:
  give dotprops.neuronlist OmitFailures argument
  more neuronlist doc fixes/additions
  missing docs for as.neuronlist.default
  add example listing all neuronlist methods
  docs: small fixes to dotprops docs
  Give mirror.neuronlist OmitFailures arg
  doc: mirror(.neuronlist) doc fixes
  Give nlapply a subset argument
fix subset.neuronlist handling of numeric indices

* fixes #142