BATCH-2041: Add support for reading excel files

This commit adds support for reading Excel files. Both JExcel and Apache POI are supported. Support for reading Excel files has been designed similar to the support for reading flat files.

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We get excel files with multiple sheets, each sheet representing a different datatype. Although probably most users only have a file with a single sheet, I think it is worthwhile to have the opportunity/ability to be able to read multiple sheets.