Comoving vs proper coordinates when depositing gas density field in EnzoMethodPmDeposit

[stefanarridge]

In lines 379-454 in src/Enzo/enzo_EnzoMethodPmDeposit.cpp, the gas density field is deposited onto the total density field. This is done by calling the Fortran function dep_grid_cic. In particular, there is some confusion about the arguments dtf, hxf, hyf, hzf, which are passed via their addresses. Specifically, in the current implementation, dtf takes the value alpha_, which is a parameter with a fixed value rather than a physical time step, and hxf, hyf, and hzf are proper (as opposed to comoving) quantities, since they are intiialised to the values of hx, hy, hz, which are multiplied by the cosmological scale factor (cosmo_a) earlier in the same block of code.

It is currently unclear whether this implementation is correct. A way of testing this could be via the Zeldovich Pancake test, as suggested in #123