Limit star formation plus outflow rate to be less than or equal to available gas mass

[bretthandrews]

If you use a high SFE, then it is possible to have a SFR plus outflow rate that exceeds the available gas mass. Do a check for this before doing the Poisson draw from the IMF.

If SFR + outflow rate > available gas mass, then redraw.

[bretthandrews]

Limit the SFR + outflow rate to be less than the gas mass / time step length. If the Poisson draw from the IMF over-consumes gas, then deterministically populate the IMF by rounding the expectation value of the number of stars in each mass bin down to the nearest integer. This should not make much of a difference in the vast majority of simulations because the numbers will be much larger than 1.