Minimal testcase with multiple runs and seed

[andsor]

This extends #12. simkernel should supply an individual seed to each run if requested by the user -- by supplying a seed argument to the simulation function (for the individual seed) and a seed argument to the call as the master seed. The library uses the master seed to generate the individual seeds.

from simkernel import Experiment

def sim(seed):
    return seed

xp = Experiment(sim)
# the seed argument is the master seed used to generate the individual seeds
res = xp(repeat=3, seed=42)

# check for uniqueness of return values
assert np.size(res) == 3
np.testing.assert_array_equal(np.sort(res), np.unique(res))