Added support for shared formula to reader

Created originally on Bitbucket by felipeochoa (Felipe)

Was already merged in Bitbucket before import, marked as merged by the import user

Source changeset not longer availableat the time of import. Commit listwill look empty

The current pull request modifies the reader to add support for shared formulae. It includes a new module openpyxl.formula.translate that uses the previously included openpyxl.formula.tokenizer to translate formulae across cells. The only other change was to the openpyxl.reader.worksheet.WorkSheetParser class, which uses the new translator module to fill in values for cells in a shared formula block. Now cells using shared formulae will not have their values set to '=', but rather to the appropriately shifted formula from the reference cell. I have not removed any of the old shared formula code, and have not modified the writer.

All tests are passing with tox openpyxl.

As before, the API documentation for the new module has been excluded (keeping it internal only for now). I could see value in turning the translator into a public API, since it would allow easier re-use of formulae in generated workbooks, but for now I'd rather keep it a private API.

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