allow xform.neuronlists with multiple registrations

* these will be applied to all neurons
* test for this behaviour
Give nmapply a subset argument - adds X to sig

* this involves adding a named arg X into second place, but that
  is no problem as we insist on the second arg being a
  neuronlist anyway.
* fixes https://github.com/jefferis/nat/issues/196
remove warning about missing args from nmapply

* at least for testing this is useful
implement Vectorisation of registrations in xform.neuronlist

* right now this only makes sense for xform.neuronlist, though
  could be some interest for single points as well I suppose
* includes test
doc clarifications
Merge branch 'feature/xform.neuronlist-multireg'

* closes https://github.com/jefferis/nat/issues/97
* nb this does not implement generic compound registrations, but this
  is already supported for cmtk registrations and in any case not
  required for this issue, just a related issue.
* feature/xform.neuronlist-multireg:
  doc clarifications
  implement Vectorisation of registrations in xform.neuronlist
  remove warning about missing args from nmapply
  Give nmapply a subset argument - adds X to sig
  allow xform.neuronlists with multiple registrations