Preserve the datatype of time arrays converted with Unit.
Merge pull request #1385 from pelson/unit_convert_dtype

Preserve the datatype of time arrays converted with Unit.
Updated whatsnew for v1.7.2 release4
Merge pull request #1388 from dkillick/whatsnew-v1.7.2

DOC: Updated whatsnew for v1.7.2 release
Bump version to v1.7.2-rc.1
Merge pull request #1389 from dkillick/v172-rc1

Bump version to v1.7.2-rc.1
Bump version to v1.7.3-DEV
Merge pull request #1390 from dkillick/v173-dev

Bump version to v1.7.3-DEV