Decreased precision in dumped forces

In this manner single and double precision tests give
consistent results.

regtest/rt2/config

 type=driver
 # this is to test a different name
-arg="--plumed plumed.dat --stride 10 --timestep 0.005 trajectory.xyz --dump-forces forces --dump-forces-fmt=%8.5f"
+arg="--plumed plumed.dat --stride 10 --timestep 0.005 trajectory.xyz --dump-forces forces --dump-forces-fmt=%8.4f"