Add smaller (more typical) GROMACS benchmark

[aturner-epcc]

The large GROMACS benchmark may not be representative of GROMACS performance for typical jobs (although it allows for scaling out to higher core counts). We should add a more typical GROMACS benchmark, for example GluCl:

http://www.gromacs.org/About_Gromacs/Benchmarks

[aturner-epcc]

Using 1400k atom benchmark from HECBioSim: http://www.hecbiosim.ac.uk/benchmarks