Add a disk usage plugin

This PR supersedes both gh-2945 and gh-2960.

The main part of this PR utilises the work in gh-2919 to provide a summary of disk usage by object type:
```
bin/omero fs usage Experimenter:0 --report
Using session dc81f7a7-9df2-4a4c-914f-67a9303bbd0b (root@localhost:4064). Idle timeout: 10.0 min. Current group: system
Total disk usage: 77381259 bytes in 99 files
 component    | size (bytes) | files 
--------------+--------------+-------
 Thumbnail    | 19923        | 16    
 Job          | 238338       | 5     
 Annotation   | 36083        | 2     
 FilesetEntry | 76817458     | 60    
 OriginalFile | 305540       | 18    
```

Part of this PR provides a method to remove the sorting from the cli usage when an incorrect option is given. This is useful order of the choices can be significant. See:https://github.com/openmicroscopy/openmicroscopy/pull/2945#issuecomment-52752080 The only subcommand using this method is `fs usage`, there the `--units` should be ordered by size while all other cli commands should be unaffected. To test try:
```
bin/omero fs usage Experimenter:0 --units Z
```
and, at least,
```
bin/omero foo
```

This PR has made some potentially significant changes on the `fs` command. The other subcommands all appear to work as before and all relevant unit and integration tests pass. However, part of testing this PR should involve checking the other `fs` subcommands.

TODO: Help comments and unit/integrations tests are still to be added as well as some changes to functionality following the conversation in gh-2945