XLSX reader: support formula cells

[lukavdplas]

Say a user has written a spreadsheet like this:

X	Y
1	=A2*2
2	=A3*2
3	=A4*2

Right now, the XLSXReader will output the values of column Y as '=A2*2', '=A3*2', '=A4*2', i.e. the formula strings. You would probably want the column to be read as 2, 4, 6, i.e. the calculated values.

Suggested implementation

Looks like this is fixed if you use data_only=True when calling openpyxl.open_workbook. Not sure if that affects anything else, though. Also, perhaps there are cases where the user does want the original formula?

[jgonggrijp]

Don't support it and tell users to convert to CSV first?