cast, join and reduce data with reshape
===

I've got a dataframe that looks like

    MAP  name       series    ID
    1.0  aspartame  baseline  902349
    1.0  aspartame  baseline  82749
    1.0  aspartame  baseline  928542
    1.0  aspartame  next      928542
    0.8  aspartame  next      82749
    0.8  aspartame  next      902349

And I'd like to join this data on ID and calculate the difference
between baseline and next in the MAP score. so this should give

    diff name       ID
    0.0  aspartame  902349
    0.2  aspartame  82749
    0.2  aspartame  928542
