fs usage: don't overwrite size (Fix #243)

[joshmoore]

see #243

  File "/tmp/omero/lib/python3.7/site-packages/omero/plugins/fs.py", line 945, in _usage_report
    if args.report and not args.size_only and size > 0:
TypeError: '>' not supported between instances of 'str' and 'int'

[manics]

$ omero fs usage --human-readable --report
Using session for root@localhost:4064. Idle timeout: 10 min. Current group: system
Total disk usage: 380.0 KB bytes in 75 files
 user | group | component    |     size | files
------+-------+--------------+----------+-------
 0    | 1     | OriginalFile | 370.1 KB |    57
 0    | 0     | OriginalFile |   9.9 KB |    18
(2 rows)

$ omero fs usage --report
Using session for root@localhost:4064. Idle timeout: 10 min. Current group: system
Total disk usage: 389153 bytes in 75 files
 user | group | component    | size (bytes) | files
------+-------+--------------+--------------+-------
 0    | 1     | OriginalFile |       379024 |    57
 0    | 0     | OriginalFile |        10129 |    18
(2 rows)

$ omero fs usage
Using session for root@localhost:4064. Idle timeout: 10 min. Current group: system
Total disk usage: 389153 bytes in 75 files

$ omero fs usage --human-readable
Using session for root@localhost:4064. Idle timeout: 10 min. Current group: system
Total disk usage: 380.0 KB bytes in 75 files