Introduce multicompartment models

[miladh]

Current branch multicompartment has some features implemented. The following needs to be done before merging with dev:

• [x] Split currents into separate elements (like LeakCurrent)
• [x] Decide on what compartments and currents should look like in GUI
• [ ] Design connection without connector dot
• [ ] Radius and length as part of compartment, automatic calculation in connection
• [ ] Measuring currents from each compartment
• [ ] Test connections between multicompartment and IF neurons (so it makes some sense)
• [ ] Verify that our model fits with NeuroML (see photonflow for parser)