Reorganized tasks in their own folder

Drafting this to propose a reorganization of the project layout:

Before:

• tasks
  • single_detector_fit.py
  • deadtime_correction.py
  • utils
    • correction.py
    • fitting.py
    • io.py
    • nexus.py

After:

• tasks
  • single_detector
    • __init__.py <-- Contains the task
    • io.py
    • utils.py <-- Previously fitting.py
  • deadtime_correction
    • __init__.py <-- Contains the task
    • utils.py <-- Previously correction.py
  • nexus.py <-- Stays at root as used in both tasks

This is mainly to separate utils so that it is easier to see in which tasks they are used.

Tell me what you think about it or if you prefer the current layout.