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