Fix IndexError in sky model analysis
This MR fixes an error with the analysis of sky models that results in an error message like:
INFO - rapthor:field - Updating sky model...
INFO - rapthor:field - Analyzing sky model...
File "/project/rapthor/Software/rapthor/venv/lib/python3.10/site-packages/lsmtool/operations/_meanshift.py", line 89, in run
weights *= self.fluxes[idx_neighbours]
IndexError: index 1 is out of bounds for axis 0 with size 1