v2.1.1

A patch release; the public API is unchanged from 2.1.0. See the validation record.

Fixed

• XLSX shared-formula followers now move every relative reference by their offset from the master, not only
  unqualified cells: whole rows and columns (SUM(D:D) → SUM(E:E), as Excel shows) and sheet-qualified cells
  (Data!A1 → Data!A2). A reference moved off the sheet becomes #REF!. Because followers are written back as
  ordinary formulas, this also corrects files rewritten from such workbooks.