Support customization of FormulaEvaluator in PoiSheet

[sravaniv-3cortex]

Our use case of reading Excel files requires that errors in resolving the external sheet references in formula evaluations be ignored. This requires calling the org.apache.poi.ss.usermodel.FormulaEvaluator.setIgnoreMissingWorkbooks(boolean). This isn't currently possible with the current PoiSheet implementation - PoiSheet is package-private so it can't be extended to override the getRow method, which allows us to customize the FormulaEvalutor instance used.

What is the feasibility of doing one of these things?

• Opening up PoiSheet for extension
• Have a settings class which can be passed by the user and later applied by the PoiSheet to the FormulaEvaluator. This might result in future asks for setting classes for other things.

If nothing above works, the other option is to duplicate PoiSheet with necessary changes and then override PoiItemReader.

[mdeinum]

While working on this I wonder if it would be a sensible default to set this to true by default. Or maybe even take it a step further and when we encounter a formula cell to always return the cached value. As we are reading the files the cached value should, generally speaking, be the right value. We don't inline change related cells or workbooks.

So from a performance perspective it might be even beneficial to, by default, ignore the formula(s).