- Make reversal potential `e` a global parameter of `pas` to avoid inconsistency 
  (reversal potential is represented by a RANGE parameter, despite it not being a conductance multiplier see #1052)
- Adjust default value of `e` to -70mV (as in Neuron)
- Streamline docs,  which use `passive`, `el`, and `gl` instead of `passive`, `e`, and `g`.

Closes #1052