Feature-based task generation cleanup

[uchchwhash]

• Remove tile_index, crs_txt, geom_feat, extra_fn_params from StatsTask
• Add spatial_id to StatsTask (replaces tile_index and feature_id)
• Add feature to StatsTask (one object that holds all relevant data)
• Rename improper name tile_index to chunk in the output_driver module
• Fix tests

[codecov-io]

Codecov Report

Merging #156 into master will increase coverage by 0.41%. The diff coverage is 54.76%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   52.85%   53.27%   +0.41%     
==========================================
  Files          16       16              
  Lines        2380     2365      -15     
==========================================
+ Hits         1258     1260       +2     
+ Misses       1122     1105      -17

Impacted Files	Coverage Δ
datacube_stats/utils/tide_utility.py	15.62% <18.18%> (+3.2%)	:arrow_up:
datacube_stats/main.py	39.91% <25%> (ø)	:arrow_up:
datacube_stats/models.py	80% <80%> (-0.77%)	:arrow_down:
datacube_stats/tasks.py	75.97% <82.35%> (+4.85%)	:arrow_up:
datacube_stats/output_drivers.py	65.39% <87.5%> (-0.29%)	:arrow_down:

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