xnat object extra features

[mmodat]

We need to add:

1. a way to skip scans with existing annotations
2. a way to store that scans should be skipped even without annotations
3. a way to download annotations
4. a way to specific whether scans should be skipped or their annotation edited

proposed

1. simply query for existence of file in the next()
2. potentially use a code in the note attribute of the scan data type?
3. rest call
4. bool in xnat object constructor - need to consider store new annotations rather than overwriting

@dadrobny Anything else?

[dadrobny]

Check:

1. XNAT default behaviour when multiple (annotation) files with the same name are uploaded.
2. Are potential error messages forwarded?