average and median file size

in `~ctbrown/scratch/2022-magsearch-benchmarking`,
```
%  scripts/intersect-ls-and-catalog.py data/ls-wort.txt data/metagenomes-catalog.txt
loaded 767277 accessions from 'data/metagenomes-catalog.txt'
example acc: ERR1083538
Loaded 767277 file sizes from 'data/ls-wort.txt'
average file size: 9716.0
median file size: 570
```