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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Given the fact that `FormulaEvaluator` caches previously calculated fields, do you think it makes sense to either allow that cache to be used or not by configuration?  And in either scenario, do we need to get a new `FormulaEvaluator` with each cell that has a formula or is something we want to get once and then just clear the cache each time if it has been configured that way?