lawn v0.3.0

NEW FEATURES

• Updated to using turf.js v3.5.2 (#49) (#50), with the following significant
  changes. Defunct functions (these methods were removed from turf, so
  made sense to remove them here): lawn_aggregate, lawn_jenks,
  lawn_quantile, lawn_reclass, lawn_size. New functions (these methods
  were added to turf): lawn_circle, lawn_collect, lawn_bbox,
  lawn_feature, lawn_tesselate. New data functions
  to create geojson: lawn_multipoint, lawn_multipolygon,
  lawn_multilinestring, lawn_geometrycollection.
• as.turf gains S3 methods for multipoint, multipolygon,
  multilinestring, and geometrycollection.
• lawn_featurecollection gains a default S3 method to error
  gracefully on classes not supported, and gains S3 methods for multipoint,
  multipolygon, multilinestring, and geometrycollection.
• lawn_erase renamed to lawn_difference, following turf changes
• lawn_average parameter name changes: field to in_field,
  outputField to out_field
• lawn_count loses parameter countField, and gains parameters
  in_field, and out_field (with default 'count')
• lawn_deviation parameter name changes: inField to in_field,
  outField to out_field (with default 'deviation')
• lawn_max parameter out_field with new default value 'max'
• lawn_median parameter out_field with new default value 'median'
• lawn_min parameter out_field with new default value 'min'
• lawn_sum parameter out_field with new default value 'sum'
• lawn_variance parameter out_field with new default value 'variance'
• all aggregatation/math functions (lawn_average, lawn_count, lawn_deviation,
  lawn_max, lawn_median, lawn_min, lawn_sum, lawn_variance)
  use turf.collect internally, same method that's used in lawn_collect

MINOR IMPROVEMENTS

• Fixed example for lawn_tag() that wasn't working. Also,
  poly_id parameter changed to field, and containing_polyid
  changed to out_field. (#30)
• lawn_merge uses new turf method turf.union internally, but
  no user facing changes
• Changed x parameter to coordinates in lawn_point for
  consistency with similar methods
• Changed rings parameter to coordinates in lawn_polygon for
  consistency with similar methods
• Internal changes to lawn_remove following turf changes, but
  there should be no user facing changes

BUG FIXES

• Fixed bug in lawn_buffer() (#53)
• Fixed bug in lawn_random(). num_vertices parameter wasn't working (#44)