Task directives do not update unless the task failed

[bryce-turner]

This was originally mentioned in #117, but the issue proposed adding reset to the task identity.

Originally from #117 :

Reset directive

This one is more difficult. There is a problem currently with not being able to update task directives unless the task identity has changed. Maybe a better fix here would be to change the workflow mash function so that it always updates task directives, but only resets the task if the identity has changed.

Alternatively, we could just add the reset directive to the identity calculation. The big downside here is that if you have a workflow already run, adding reset directives to a task will reset it.

This mostly focused on the reset directive since we may want tasks to reset, but we don't necessarily want to update the task identity and therefore cause more tasks to restart than needed.

Another example is say we want to add an input or output to a task for better support with the slurm_singularity backend. Right now we would have to set the task to failed or otherwise force it to be re-rendered.