Change 'states' --> 'initial_states' when initializing model

[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

The input argument of class JumpProcess and ODE named "states" has been renamed to "initial_states" because this is more descriptive.

So to initialise an SIR model use,

model = SIR(initial_states={'S': 1000, 'I': 1}, parameters={'beta': 0.35, 'gamma': 5})

Instead of,

model = SIR(states={'S': 1000, 'I': 1}, parameters={'beta': 0.35, 'gamma': 5})

One advantage is that previously, the "states" were renamed "initial_states" upon model initialization. So if you want to access the initial states of your model you'd have to model.initial_states but the input argument was named "states". The new implementation is more consistent.