Fixed scenario excel import for numbered scenarios

[mrvisscher]

Fixes a bug where AB throws an error when importing an excel scenario file that contains scenarios that are called numbers. This only fails for .xlsx imports, not .csv imports.

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [x] Update the documentation, please follow the numpy style guide.
• [x] Update tests.
• [x] Categorize the PR by setting a good title and adding one of the labels: bug, feature, ui, change, documentation, breaking, ci as they show up in the changelog.
• [ ] Link this PR to related issues by using closing keywords.
• [x] Add a milestone to the PR for the intended release.
• [x] Request a review from another developer.

[coveralls]

coverage: 50.688%. remained the same when pulling eb2d49c1177417d15964f462ac1e6ebd4cd09ea4 on mrvisscher:numbered_excel_scenarios_fix into 0955df9ad3644251c9511f73a3998cdefc99df74 on LCA-ActivityBrowser:master.