Export data layout field to xlsx correctly.

Excel uses a field index of -2 to indicate a data layout field.

Change-Id: I6b18c0bcff439eb4425ef0b0d0b841633dc81dd7
Use the source dimension name when searching for a dimension.

Otherwise we might miss the right dimension object.  This fixes the bug where
the subtotal function of the second data field was not set correctly when
importing from xlsx.

Change-Id: Id6ecb07b86cf6803a3f6f7604267ce2f5f9a4067
Correctly export subtotal functions of data fields.

Take note that data field may be duplicated.

Change-Id: I8f787075869f38d0101da2787bac315c71d8a6e8
Write test for importing and exporting of pivot table with 2 data fields.

This is for xlsx format.

Change-Id: I8ef2ae41cc88eeeab610e1a89726c62acb9e2fcb