Allow for non standard cosmologies, and improve cosmological time integration #711

Job	Run time
Build & Lint: HIP TYPE=hydro	2m 31s
Build & Lint: HIP TYPE=gravity	2m 40s
Build & Lint: HIP TYPE=disk	3m 2s
Build & Lint: HIP TYPE=particles	2m 57s
Build & Lint: HIP TYPE=cosmology	3m 2s
Build & Lint: HIP TYPE=mhd	2m 29s
Build & Lint: HIP TYPE=dust	2m 32s
Build & Lint: HIP TYPE=cooling	2m 45s
	21m 58s

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