Add changes to initial conditions to draw functions

[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

Previously draw_functions could only be used to update model parameters between consecutive runs, they always needed to have parameters as the first argument,

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

I've extended draw_functions to be able to alter the initial condition between consecutive runs,

def draw_function(parameters, initial_states):
    parameters['gamma'] = np.random.uniform(low=0, high=5)
    initial_states['S'] = np.zeros([5, 11])
    return parameters, initial_states

You must now always provide parameters and initial_states to the draw function as mandatory arguments. For initial states, the size of the states is checked.

The goal of this change is to build towards the ability to optimisedraw_function_kwargs using pySODMs optimization module. This could be interesting to calibrate the initial condition of epidemic models.