lognorm_im fails with callable par

[cliffckerr]

Test script:

import starsim as ss

which = 0 # Choose 0 (works) or 1 (fails)

def return_ten(*args, **kwargs):
    return 10

dist = [
    ss.lognorm_ex(mean=return_ten),
    ss.lognorm_im(mean=return_ten),
][which]

pars = dict(
    diseases = dict(
        type = 'sir',
        dur_inf = dist,
    ),
    networks = 'random',
)

sim = ss.Sim(pars)
sim.run().plot()

[cliffckerr]

Closed by #581