Follow-up to #438

How should we deal with Pumps? Currently, they are separate from the Channel. We could also merge them and have something like self.ion_name=None (or self.ion_name = "v") for Channel`s.

Currently, we define self.pumps and self.pumped_ions. As @jnsbck suggested, we could also define self.pumps = {"ion_name": [pump1, pump2]}. If we merge Pumps and Channels, we could even have:

self.mechanisms = {"v": [channel1, channel2], "ca": [pump1], "na": [pump2]}

A downside of this is that

1. by removing the Pump, we lose the opportunity to write explicit docstrings that explain what a Pump is
2. we hide that a Pump and a Channel differ in their units and in the fact that channels get the current divided by the capacitance.

A final option would be to only go for the dict representation for Pumps. This has the downside that:

1. we now handle pumps and channels differently
2. we can no longer do self.channels + self.pumps

But it has the big advantage of having only one attribute (get rid of self.pumped_ions)