Add data download to plugins

It would be useful to have data download in many of the plugins.
E.g. InplaceVolumes and the sensitivity plugins.
Some discussion might be needed case by case for what data should be downloaded, i.e. all data or only the data visualized.

Start by reviewing the InplaceVolumes and sensitivity plugin entry points to understand how each exposes data. Clarify whether downloads should include all plugin data or only visualized data, and whether the behavior differs by plugin. Done means the download scope is agreed case by case and implemented across the selected plugins.

Written by the indexing model from the issue text.