allow individual task times for workflow tasks or specfem executables

[bch0w]

Currently there is only one tasktime that submits the walltime for a given job (e.g., forward simulation, smoothing). This value is not uniform for all tasks (e.g., adjoint simulation is longer than fwd), so we end up needing a maximum time required. Usually this is the smoothing code, or adjoint simulations. It would be useful to be able to set longer task times for tasks which take a long time (e.g,. gaussian smoothing takes 5--10x longer than forward simulations), or maybe do away with task times altogether? I think some clusters require it, or base their job priorities on it, so it's probably important. Some simple architecture would be to set tasktime as a dictionary, with one 'default' value and then if the User wants to override, they can match function names from Workflow.task_list() to new task times based on how long they expect that function to take.