Calculate change for 2010 fields which don't have exact 2000 equivalents

[onyxfish]

Sum fields can be summed/subtracted/etc to make comparable fields.

[curtmerrill]

There is a file in the import/fieldkeys folder that pairs 2000 and 2010 keys.

If there is no analogue 2000 field, the column is blank. For some fields, there is a 2000 field with no analogue 2010 field, so the 2010 field is blank, but there is still a label.

A few fields require addition or subtraction, and the addition/subtraction occurs in both 2000 and 2010 columns.

[onyxfish]

Pushing this off to "Nice to have" since the basic functionality is what matters most.