Check some operations to make sure users can't create files on unassignable DPs or groups

[prioux]

We have some file operations in CBRAIN which make old assumptions: if you have access to a DP you can write to it, if you have access to a file through a group, you can assign other files to the same group. Example: the "extract" file from a collection, which will extract to the same DP and with the same group ID as the collection. This is wrong when the DP or collection is a dataset assigned to a group that is 'non-assignable'. The code should automatically switch the group and DP to another one (or show an error message to the user?)