Moral don't tidyup just before pushing

When not explicitly given checks for GSL are made anyway, when explicitly given the checks must be successful

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

Grackle chemistry --> GEAR chemistry. Moved the output of subgrid model to a separate part of the HDF5 file.

We might hope to be clean against all the issues it identifies

Just uses abort() instead of exit() in non-MPI code