Add duration vector type to dataframe

[kylebrandt]

Duration type will be useful eventually, good task to get a little familiar with arrow.

All in dataframe package:

• Make []time.Duration and []*time.Duration Vector types and update newVector()
• Update NewField to support the type
• Update golden arrow test file with duration and nullable duration vectors via var update and goldenDF() in arrow_test.go. See dataframe package's README for more info.
• Make two arrow column builders for these, update buildArrowColumns() type switch to use them, and update fieldToArrow(). See NewDurationBuilder
• Update the the two type switches in in UnMarhsalArrow to support this field. (May be different if someone has completed Refactor dataframe UnMarshalArrow #19). See NewDurationData