simplify calculation of inertia in dotrops

* no change in program logic
* always meant to tidy this up from Nick Masse's original
dotprops objects can contain multiple neurons
Merge branch 'master' of github.com:jefferis/nat

# By James Manton
# Via James Manton
* 'master' of github.com:jefferis/nat:
  Fix minor typo in README
Some extra docs/examples re specifying file format

* see #160
Give write.neurons a format argument

* closes #160
Add generics and methods for set operations on neuronlists
Merge branch 'neuronlist_sets' of github.com:ajdm/nat into ajdm-neuronlist_sets

* 'neuronlist_sets' of github.com:ajdm/nat:
  Add generics and methods for set operations on neuronlists
roxygenise (intersect not exported)
add ... arg to generic set methods

* we don't need it, but this is just good style for a generic
don't repeat base definition, just use it

* see e.g. https://github.com/hadley/dplyr/blob/master/R/sets.r
remove redundant @method tags

* no longer required
all set methods should implement ... argument
fix cross-references to base set functions
Merge branch 'ajdm-neuronlist_sets'

* based on ajdm/neuronlist_sets
* closes #161
* ajdm-neuronlist_sets:
  fix cross-references to base set functions
  all set methods should implement ... argument
  remove redundant @method tags
  don't repeat base definition, just use it
  add ... arg to generic set methods
  roxygenise (intersect not exported)
  Add generics and methods for set operations on neuronlists