ProcessBuilder __getitem__ support

[soxofaan]

from openEOPlatform/documentation#10

def fit_function(x:ProcessBuilder, parameters):
    pi = math.pi
    a0 = array_element(parameters, 0)
    a1 = array_element(parameters, 1)
    a2 = array_element(parameters, 2)
    return a0 + a1*cos(2*pi/31557600*x) + a2*sin(2*pi/31557600*x) # 31557600 are the seconds in one year

It would be nice to allow writing parameters[0] instead of array_element(parameters, 0)