Feature/handle excel files with incorrect file extensions

[ollietulloch]

Logic already exists to attempt to load an xlsx_file_with_xls_extension.xls, i.e. it's an excel file with the wrong excel extension.

This PR allows the NdrImport::File::Excel handler to load excel workbooks that have the incorrect file extension, for example .dat files received via Mesh. It creates a copy of the file with an updated file extension, taken from the NdrImport::Table.format. If the format provided still doesn't work, the workbook will fail to load in the usual way.

If the NdrImport::Table.format is an excel format, we do not create a new file. This causes a loop in the existing logic.

Files created are deleted once the workbook has either loaded, or failed to load.

Also included is some light refactoring/boyscouting of the load_workbook method, to appease rubocop.