Spike_threshold_PS2

Detect spike threshold in membrane potential using phase space V versus dV/dt.

Spike threshold detection alogrithm - Method II
This function calculates the spike threshold of a membrane potential
recording based on the method by Sekerli et al IEEE Trans Biom Eng
51(9): 1665-1672, 2004. In this paper they suggest two methods of
estimating threshold. Here we use the second method, which is based on
revealing the threshold in a phase plot of V versus the derivative
dVdt. The largest increase in slope is an indication of the threshold.