Address FutureWarning in isotonic regression

[nicholasloveday]

We need to deal with FutureWarnings be triggered by lines 250 and 253 in scores/scores/src/scores/processing/isoreg_impl.py

FutureWarning: In a future version of xarray the default value for join will change from join='outer' to join='exact'. This change will result in the following ValueError: cannot be aligned with join='exact' because index/labels/sizes are not equal along these coordinates (dimensions): 'stn' ('stn',) The recommendation is to set join explicitly for this case.
merged_ds = xr.merge([fcst.rename("fcst"), obs.rename("obs")])