0.5.12

• Added withSpatialIndex to introduce a column assigning a z-curve index value based on the tile's centroid in EPSG:4326.
• Added column-appending convenience methods: withExtent, withCenter, withCenterLatLng
• Documented example of creating a GeoTrellis layer from a RasterFrame.
• Added Spark 2.2.0 forward-compatibility.
• Upgraded to GeoTrellis 1.2.0-RC2.

0.5.11

• Significant performance improvement in explodeTiles (1-2 orders of magnitude). See #38
• Fixed bugs in NoData handling when converting to Double tiles.