Upgrade to pixetl fixed for rasterizing vectors

[dmannarino]

Pull request checklist

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• Make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
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Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

As before the ill-fated pixetl 1.7.5, rasterizing vector datasets can crash the DB

Issue Number: N/A

What is the new behavior?

• Rasterizing vector datasets is done more carefully, limiting parallelization when accessing the DB

Does this introduce a breaking change?

• Yes
• No

Other information

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f285d5f) 80.32% compared to head (1ca5114) 80.32%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #452   +/-   ##
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  Coverage    80.32%   80.32%           
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  Files          124      124           
  Lines         5515     5515           
========================================
  Hits          4430     4430           
  Misses        1085     1085           

Flag	Coverage Δ
unittests	80.32% <ø> (ø)

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