Output modules break parameter exploration

[remram44]

Output modules don't work with parameter explorations. This is a problem because #1017 upgrades cell modules to output modules.

@Juliana:

I tried to do a parameter exploration on terminator.vt (isosurface) and selected the time axis. The result is generated, but when I click play nothing happens.

This seem to all be going through the PipelineInspector: inspect_spreadsheet_cells() sets spreadsheet_cells.