Automatically assign project-specific log directories to tasks in the QC pipeline

[pjbriggs]

Update the QC pipeline code in qc/pipeline.py to allow a default log directory to be defined when adding each project (via new set_default_log_dir method), which is then implicitly used when tasks are added (by overriding the add_task method of the base Pipeline class).

This allows the explicit setting of the log directory to be removed from each add_task call in the add_project and related methods, reducing the amount of boilerplate code and ensuring that any new tasks which are added in future will automatically have the correct log directory assigned.