consistent type with asserts

[thonic]

if you want to use the convert_continuous_to_discrete function with the rest of the models, such as:
pyhawkes.models.DiscreteTimeNetworkHawkesModelSpikeAndSlab.add_data()
it will now scream

"Data must be a TxK array of event counts"

because of assert S.dtype == np.int

this changes the returned type so that it is consistent with the rest of the code

[slinderman]

Thanks!