Bugfixes for elasticsearch filtertransformer comparision operators.

[markus1978]

With the current elasticsearch filtertransformer some of the optimade-validator checks fail due to buggy comparison operators. This fixes it.

[codecov]

Codecov Report

Merging #648 (d546133) into master (ffbe0fb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #648   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files          60       60           
  Lines        3257     3257           
=======================================
  Hits         3038     3038           
  Misses        219      219           

Flag	Coverage Δ
project	93.27% <100.00%> (ø)
validator	66.13% <0.00%> (-0.77%)	⬇️

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Impacted Files	Coverage Δ
optimade/filtertransformers/elasticsearch.py	87.97% <100.00%> (ø)

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[ml-evs]

Thanks for this @markus1978!

I'm going to be adding elasticsearch to the CI as a proper backend between #613 and a future PR. (see my comment in #339, apologies for closing that one early!)