Use a MultiIndex to index the optimal block dataframe.
Use a MultiIndex to index the optimal block dataframe.

Previously we concatenated the variables to form a string of the form
'<file> <calc number>' but this caused output to be ordered
lexicographically so '<file> 10' occurred after '<file> 1' rather than
'<file> 9'.  Using a MultiIndex fixes this automatically.

hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande