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

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide.
• Update tests.
• 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.
• Add a milestone to the PR for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 50.688%. remained the same
when pulling eb2d49c on mrvisscher:numbered_excel_scenarios_fix
into 0955df9 on LCA-ActivityBrowser:master.