spatial_constant subexpression in SpatialNeuron
LFP example running! Looks ok (but I don't have a reference for now).
Merge remote-tracking branch 'remotes/origin/master' into spatialneuron
SpatialNeuron.space_constant now works with subgroups
Code for the sharpness paper (PLoS CB 2013)
Added units checks in Morphology
SpatialNeuron: internal variables are now state variables (cleaner)
Merge remote-tracking branch 'remotes/origin/master' into spatialneuron

Conflicts:
	brian2/spatialneuron/morphology.py
	brian2/spatialneuron/spatialneuron.py
	examples/compartmental/cylinder.py
	examples/compartmental/infinite_cable.py
	examples/compartmental/lfp.py
	examples/compartmental/rall.py
Some more minor example cleanup
Pass an empty `user_code` argument when getting subexpressions

This presents a warning if a subexpression refers to variable names that
are also present in the local namespace.
Raise an `IndexError` for too high integer indices in `Morphology`

Otherwise iterating over a morphology (which can be done automatically, e.g.
when trying to index a numpy array with it) will lead to an infinite loop
Update/clean up sharpness paper examples
Update travis.yml to latest miniconda version

(cherry picked from commit 7ed7b3cc49f3e28c564c1052695c0aa949c875c7)
Merge pull request #382 from brian-team/spatialneuron

Spatialneuron