[FEATURE] add cross-group functionality to `post_table`

[erickmartins]

Is your feature request related to a problem? Please describe.
Right now (as of PR #58), post_table() cannot do cross-group posting - that is specifically because the OriginalFile created for the file annotation that contains the table seems to always be created on the original group for the conn object, ignoring the conn.SERVICE_OPTS.setOmeroGroup() from set_group() and leading to a group mismatch between FileAnnotation and OriginalFile.

Describe the solution you'd like
Just make it work!

Describe alternatives you've considered
One alternative is trying a different solution for switching groups such as conn.setGroupForSession() and see if it makes any difference.