Increase HDF5 chunk size and set shuffle filter for single and serial output when compression is switched on.

Cosmological terms in vectorized version of Gadget SPH.

Closes #406

See merge request !510

Pass the cosmological model to the cooling function and get the physcial density and internal energy.

Cosmological time integration

See merge request !509

Re-instate the explicit casts after malloc in proxy.c that had disappeared by mistake in a bad merge. Matthieu be more carefulgit status!

Might be better to use a --with argument so that external builds can be located more easily in non-standard places