export of full system model to different formats

[fzahle]

the bootstrap-tables now offer export to a bunch of formats for single components, but there's no nice way to export the entire model and data from all sub-comps and with the new detail view of arrays and vartrees these don't get exported unless expanded and this looks clunky in Excel.

We have the full model in a dictionary generated in app.py:build_hierarchy, so it should be piece of cake to export this to formats like JSON, YAML etc. The only thing missing is Excel. The Excel workbook should have a series of tabs for the main assembly I/O followed by tabs for all sub components. Any takers?