Add Custom Nu flag

This request tackles #70 (closed). With this request, we introduce a new optional key in the stage0 called pileup-nu.

The pileup-nu key will override whatever the current Nu value is, allowing for customisation of the Nu value used in the stage6. Of course, this will fail if the option file doesn't actually exist in AppConfig. Eg. if you ask for Nu=123.4, this will fail as no such option file exists.

The key is set to the same level as datatype, within samples:

samples:
  - event-types:
      - 23103006
    data-types:
      - 2022
    num-events: 10
    pileup-nu: 2.1
    magnet-polarities:
      - MagDown

TODO:

• Remove final printout and comments
• Add reference for this
• Change name?

Closes #70 (closed)