Distinguish resting potential from reset voltage

[Yanqi-Chen]

For simple models such as IFNode, this will not induce much difference. However, for LIFNode and even more complicated neurons (e.g. adaptive neurons), splitting up them induces more variants of dynamics.

[Yanqi-Chen]

To ensure forward compatibility, the default value of v_rest can be set to the same as the v_reset.

[tmasquelier]

Good enhancement!!!