Cosmology radiation transport GEAR-RT

First steps towards a cosmological treatment of radiative transfer with GEAR-RT

Changes:

• Added cosmological timestep in rt_part_dt()
• Added function to write (comoving) photon energy densities to hdf5 in rt_io.h
• Changed Gizmo scale-factor exponent for mass from 1.f to 0.f

Examples added:

• Cosmo_Advection_1D: Test that the propagation speed of photons is correct. Also tests that the physical energy density is diluted with correct scale factor
• Cosmo_Advection_2D: Test that the propagation speed of photons is correct. Also tests that the physical energy density is diluted with correct scale factor
• Cosmo_uniform_box_3D: Test that comoving and physical energy + energy density scale correctly with scale factor
• Cosmo_StromgrenSphere_3D: Test that the front of the radiation sphere propagates with c

added radiative transfer label

requested review from @mivkov

assigned to @mivkov

Just flying by to say I like to see a code push request. Ping me if/when you need me to do something.