Run best should be able to find the executable

Right now the executable for `run_best` of `propti_analyse` looks for a relative path from the directory of the current best parameter set in the analysis directory. An absolute path might be better to avoid errors in finding the executable.

e.g. eariler I've changed the location of where the current best parameter set is written but didn't change the (hard coded) relative path accordingly, thus the simulations cannot be conducted.