[BUG] sire.load will hang when there is no internet connectiion and GROMACS isn't installed

[lohedges]

The sire.load function will try to work out the GROMACS share path at runtime and will download some standard GROMACS files if the path isn't found. This results in a hang if GROMACS isn't installed and the user has no internet connection. This happens even when the user explicitly passes the gromacs_path to the function, since _get_gromacs_dir is incorrectly called twice, i.e.:

sire/src/sire/_load.py

Lines 442 to 450 in 208c15f

	if gromacs_path is None:
	gromacs_path = _get_gromacs_dir()
	m = {
	"GROMACS_PATH": _get_gromacs_dir(),
	"show_warnings": show_warnings,
	"parallel": parallel,
	"ignore_topology_frame": ignore_topology_frame,
	}