Detect existing campaign

[JoelLucaAdams]

When creating a new Campaign object, the _counter is always initialised to 0, regardless of whether there are existing runs in the campaign directory. This causes previously completed runs to be overwritten if a new campaign object is created for the same directory.

This behaviour is particularly problematic in active learning workflows, where simulations are run iteratively and a new Campaign object is created after each run to generate a new input.deck.

A new function, allocate_next_run_number, has been introduced which searches all existing directories and returns the highest run number plus 1, or 0 if no runs exist. Only the lowest-level run directories are considered (e.g., 'run_0', 'run_50').

[LiamPattinson]

Maybe this could include a toggle argument append: bool = False so that the current behaviour is still the default, but users have the option of adding to an existing set of runs?

[JoelLucaAdams]

append flag has now been added which decides whether to append new runs to the end of an existing campaign (if True) or overwrite it (if False)

[JoelLucaAdams]

resolves #13