list all channels and synapses in a more organised way

[jnsbck]

add channels sorted by ions to readme.
make jx-mech.info() or similar that lists available channels and where they can be imported from + listed ions/properties/attrs.

[huangziwei]

yes, it'd be good to have a overview of all channels and synapses somewhere.

I think we can implement different things for different purposes:

1. just to list them somewhere: maybe add a channels_and_synapses_info notebook, and read all META data from them and list them in a data frame;
2. a function to find and return the specific channels in the code, e.g. find_channel(name="Na") and returns [fm97.Na, benison01.Na, ...] directly.