Possibly using variable before assignment

[ns-rse]

I touched topostats/tracing/ordered_tracing.py and pylint reported some possible problems that may come back to bite us...

topostats/tracing/ordered_tracing.py:937:40: E0606: Possibly using variable 'ordered_traces_data' before assignment (possibly-used-before-assignment)
topostats/tracing/ordered_tracing.py:945:12: E0606: Possibly using variable 'tracing_stats' before assignment (possibly-used-before-assignment)
topostats/tracing/ordered_tracing.py:948:42: E0606: Possibly using variable 'grain_molstats' before assignment (possibly-used-before-assignment)
topostats/tracing/ordered_tracing.py:958:23: E0606: Possibly using variable 'images' before assignment (possibly-used-before-assignment)

If any of the earlier stages that generate these fails then they will not exist when they are added to the various dictionaries noted at the above lines.

We probably need a try: ... except: in case any of the tracing methods fail for a given grain.