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<pre class="fragment">Main initialisation function
Note that the grid can be altered at any time using the hydro module

Parameters
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ncells: integer
    Number of cells to use
    default: 512
rmax: float
    Sets the maximum size of the grid
    default: 20 pc
n0: float
    Sets the initial Hydrogen number density of the gas
    default: 1000 cm^-3
T0: float
    Sets the initial temperature of the gas
    default: 10 K
gamma: float
    Sets the adiabatic index of the gas
    default: 5/3 (mona