Rework checks output dir
Improvement to !21 (merged) - as of that MR, the checks code will attempt to create a directory checks in the same directory as where the ntuple used as its input is stored. This works fine for local tests, but won't allow this code to be used for other applications, eg. with premade tuples stored in read-only locations.
This MR adds adds an additional argument to lb-ap check where the user must specify a location to store the output. The behaviour of lb-ap test is unchanged (ie. local tests will still automatically create the checks within the test's output folder as before).
Work related to DPA task https://gitlab.cern.ch/lhcb-dpa/project/-/issues/93.