[Feature] Multidimensional trend detection (noise responds to other columns)

[RussellSB]

• multidimensional trends: str list for val_col params.
• allow noise detec to be responsive to other column: e.g.
  • param: None is default behaviour, each trend detec is wrt itself
  • param: str value, its with respect one column in particular (e.g. response sales for interrupted time series use cases)
  • do not allow for str list here

[RussellSB]

to keep things simple can decide to loop over each trend to start with,
and will need to update the plot function to handle it. e.g. 3 value cols, should be 3 subplots.

Can consider opening a later PR after this with low priority to concern with a proper multidimensional array implementation somehow ... if necessary.

That would focus on performance rather than anything else. And will need to be assessed if it could be a practically significant performance improvement .... otherwise ignore. The algorithm is already quite efficient as is.