Notify users about new datasets on ODP

[forman]

Notify users about new datasets on ODP.

It would be very useful if Cate could tell users what changed after it has updated its local meta-data caches from ODP.

[papesci]

There is an implementation in a branch it create a freeze version of the dataset list used to compare with the most updated DS set to find differences. The differences could be new or deleted DS, thay are so saved in a third json file dataset-list-diff.json, which could be opened to verify the updates at any time. I am considering to add also an option at the cli command ds list to enable the user to print out the changes found.

[papesci]

branch merged

[forman]

Reopening this issue as it has the label "gui" which means there is work to be done in Cate Desktop - namely notifying the user. I can't see where this has been implemented.