Better matrix export of product systems

[msrocka]

Currently the matrices of a product system can be exported export into a CSV and Excel format. It would be good to harmonize this format a bit and to support all features in the exports (e.g. export of regionalized matrices, uncertainty distributions). Also, we could support the NPY format for further processing of the matrices in Python.

• [x] New export dialog
• [x] Write CSV, Excel, NPY files
• [x] Write a README to the export folder that describes the format
• [x] In case of the Python export, write an example script to the folder
• [ ] Matrices with data quality indicators

This is how the new dialog looks like: