Add a plugin for determining disk usage.

This plugin utilises the work in gh-2919 to provide a summary of disk usage by object type:
```
bin/omero usage Image:1,2,3  --report
Using session b011dfc4-2d21-4898-b72b-0a366891414a (user-0@localhost:4064). Idle timeout: 10.0 min. Current group: pg-0
Total disk usage: 153629 bytes
 component    | size  
--------------+-------
 Job          | 59124 
 Annotation   | 8     
 FilesetEntry | 90432 
 Thumbnail    | 4065  
(4 rows)
```
This is very much work in progress in both the name, `usage` is not that good, the object arguments and the output styles, more for discussion than review at this stage.

/cc @mtbc @joshmoore @sbesson

--depends-on #2919
I did wonder whether truncating, rounding or even rounding up (ie always give the worst case) was better. I'll round for now and leave it open.