Implement omega_gas
Minor cleanups
re-implement omega_DLA
Check the types and sizes of the input arrays in the C module.
Let input arrays be of type float rather than double, since that is what
is stored in the snapshots.
Fix bug in the fieldizing code where using subsampling was causing us to lose mass. Basically change = to +=
Set an exception in the right way
Remove all the gas_grid stuff - if I want separate grids, I can just create another HaloHI object