using pchip instead of spline in load_OCP.m

[dainjungdev]

pchip and spline both generate cubic polynomials.
Spline works better with smooth data. But using spline can cause abnormal voltage values outside the range(2.5-4.2V) around boundary points(around SOC = 0, SOC = 1). pchip(Piecewise Cubic Hermite Interpolating Polynomial) has no overshoots/less oscillations, so I think it is more suitable when fitting a monotonic function like SOC vs Voltage.