Add a method for bytes for a report sheet

[colanconnon]

We need a method for return the raw excel file bytes for a report sheet. Blaze utils has a method that would be help to attach to the report sheet (https://github.com/blazelibs/blazeutils/blob/65fc8f5a7386fcd7a4efe00a890659d6d1c7bef8/blazeutils/spreadsheets.py#L75)