Version 1.6.3

Minor release, mostly bugfixes. Includes contributions from @philippjfr, @ahuang11, and @rsignell-usgs.

New features:

• Add geo features for US states (#312)
• Add ESRI Ocean tile layers (#320)

Bug fixes and minor improvements:

• Add dtype methods to geometry interfaces (#345)
• Ensure that Line Shape is not filled in matplotlib (#326)
• Avoid zooming in beyond tile/axis resolution (#325)
• Fix gridlines for gv matplotlib overlay (#308)