Faraday and Ideal Gas constants are different in original model and in NeuroML

In NeuroML they are:

```
Faraday = 96485.3
R = 8.3144621
```

In original model they are:

```
Faraday = 96489
R = 8.31342
```

This probably creates some differences between the membrane potentials.

In the modified version, I set these constants to the values defined in NeuroML.
