Add gap junction support

[clinssen]

Fixes #522.

• Prototype support for neurons that are nonlinear in V_m
• Prototype support for neurons that are linear in V_m
• More precise/accurate numerical testing (analytical solution for a two-neuron system?)

Suggested reviewers: @bstrohmer

[bstrohmer]

Using the first test cases suggested by Hahne et al., 2015 in "A unified framework for spiking and gap-junction interactions in distributed neuronal network simulations", I was able to confirm the implemented gap junctions promoted synchrony as desired between coupled single neurons and neural populations.

[clinssen]

Thank you!