Give plot3d.neuronlist a SUBSTIUTE argument

* this allows other functions to control non-standard evaluation

see http://adv-r.had.co.nz/Computing-on-the-language.html#calling-from-another-function

for discussion
Test controlling non-standard evaluation in plot3d.neuronlist

* as other functions might do
Give plot.neuronlist a SUBSTITUTE argument as well
remove non-existent parent.generations argument

* closes #140
Pass dots extra args to filterfun

* closes #139
Merge branch 'feature/nse-backdoor'

* feature/nse-backdoor:
  Pass dots extra args to filterfun
  remove non-existent parent.generations argument
  Give plot.neuronlist a SUBSTITUTE argument as well
  Test controlling non-standard evaluation in plot3d.neuronlist
  Give plot3d.neuronlist a SUBSTIUTE argument

* needed for jefferislab/nat.nblast/#11
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