Adds formatting to get_last_granule and replaces fill value with Empty

get_last_granule attempts to stringify values from recent granules of a
dataset. Currently the values are displayed as-is, this patch replaces
fill values with the word "Empty" to signify there is no value there and
it formats time stamps into ISO8601 Zulu-time.
Merge pull request #685 from lukecampbell/dp_strings

Adds formatting to get_last_granule and replaces fill value with Empty