POT_ARR/E_R outside Plasma

[kudav]

When supplying the electrostatic potential as input through POT_AUX, the values outside the plasma (i.e. S>1) are initialized to 0. This leads to unphysical spikes in E_R when the potential has finite value at the edge (see below). An easy fix would be to set POT_ARR=POT_AUX_F(end). Ultimately, the extrapolation behavior of the splines should be changed, as then POT_AUX could also be supplied for S>1 with consistent behavior.