Utility to convert shapes to GeoJSON

[halfabrane]

adds a utility magellan.GeoJSON, along with a Spark SQL expression to convert shapes to GeoJSON geometries. A shape of the form Point(0.0, 0.5) will be converted to

{"type": "Point", "coords": [0.0, 0.5]}

Usage:

import magellan.GeoJSON
GeoJSON.writeJson(shape)

Usage in Spark SQL:

import org.apache.spark.sql.magellan.dsl.expressions._
df.withColumn("json",  $"point" asGeoJSON)

[codecov-io]

Codecov Report

Merging #197 into master will increase coverage by 0.26%. The diff coverage is 93.65%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   89.57%   89.84%   +0.26%     
==========================================
  Files          48       49       +1     
  Lines        1564     1605      +41     
  Branches      103      112       +9     
==========================================
+ Hits         1401     1442      +41     
  Misses        163      163

Impacted Files	Coverage Δ
src/main/scala/magellan/GeoJSONRelation.scala	100% <ø> (+11.11%)	:arrow_up:
src/main/scala/magellan/geojson.scala	100% <100%> (ø)
src/main/scala/magellan/dsl/package.scala	82.35% <50%> (-4.32%)	:arrow_down:
...che/spark/sql/catalyst/expressions/functions.scala	95.55% <86.36%> (-1.79%)	:arrow_down:

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