stochastic option in config

[alsnhll]

The option to run stochastically vs deterministically should not be an environmental variable/command line input only but should also be in the config under integration::method (in addition to the deterministic methods rk4 and legacy)

[shauntruelove]

I agree with this. I added it as you said to the config, replacing rk4 in method

seir:
integration:
method: stochastic
parameters:
sigma:
...

Need to add it to the code to read the option still.