Open xlsx files

Summary

pyspread should be able to read xlsx files. The development is taking place in the pycel branch.

To Do

• Import cell values
• Convert xlsx formulas to Python code
• Make absolute references work
• Gather Sheet dimensions and no. sheets and adjust shape
• Make references across sheets work work
• Apply cell styles
• Apply merged cells
• Make relative references work (After close inspection of the pycel code, relative references seem to be infeasible with pycel.)
• Convert Pivot tables (It ist still unclear how this could be achieved with the current frameworks.)
• Convert cell formats to object types for date, time and currencies (The ambiguities in Excel's approach makes this hard to work correctly in all cases. Maybe type conversion could be added later versions.)