Draw functions with flexible number of parameters

[twallema]

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have updated the documentation accordingly.

Describe your fixes/additions/changes

Old

Draw functions with two mandatory inputs: parameters and samples, even if samples wasn't used,

def draw_function(parameters, samples):
    parameters['alpha'] = np.random.uniform(low=0, high=5)
    return parameters

out = model.sim(121, N=10, draw_function=draw_function, samples={}, processes=10)

New

Only parameters is mandatory,

def draw_function(parameters):
    parameters['alpha'] = np.random.uniform(low=0, high=5)
    return parameters

out = model.sim(121, N=10, draw_function=draw_function, processes=10)

But users can add any number of arguments, and need to supply them to the sim() function with the argument draw_function_kwargs (type dict, keys are names of extra args),

def draw_function(parameters, low, high):
    parameters['alpha'] = np.random.uniform(low=low, high=high)
    return parameters

out = model.sim(121, N=10, draw_function=draw_function, draw_function_kwargs={'low': 0, 'high:5}, processes=10)

@RitaVazVerstraeten Check any draw functions in your code if using them.