Provides a flag to control turning on auto indexing of spatial relations

[halfabrane]

use option magellan.index = true while defining the spatial format to trigger auto indexing of the dataset. The precision of the index can be set using magellan.index.precision
eg.

spark.read.format("magellan").
  option("magellan.index", "true").
  load(path)

triggers a ZOrder Curve based index with default precision of 6 characters.

spark.read.format("magellan").
  option("magellan.index", "true").
  option("magellan.index.precision", "35")
  load(path) 

triggers a ZOrder Curve based index with 7 character precision

[codecov-io]

Codecov Report

Merging #137 into master will increase coverage by 2.8%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #137     +/-   ##
=========================================
+ Coverage   82.62%   85.43%   +2.8%     
=========================================
  Files          48       47      -1     
  Lines        1502     1359    -143     
  Branches       98       92      -6     
=========================================
- Hits         1241     1161     -80     
+ Misses        261      198     -63

Impacted Files	Coverage Δ
src/main/scala/magellan/SpatialRelation.scala	100% <100%> (ø)	⬆️

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