Refactor ClickHouseChain for efficient slice selects

[michaelosthege]

• Returning empty arrays (of correct shape and dtype) was needed because slices can result in an empty selection.
• Increased coverage of the slicing test cases, including empty slices, reverse direction slices and dynamically shaped variables.
• Implemented WHERE clause for the ClickHouse queries to download only the needed elements. This uses >=, < and a modulo operator.

The first CI run was ❌ just because of a too-low coverage.

[codecov-commenter]

Codecov Report

Merging #51 (0c70c3b) into main (1b2fe3b) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   95.80%   95.93%   +0.12%     
==========================================
  Files           8        8              
  Lines         572      590      +18     
==========================================
+ Hits          548      566      +18     
  Misses         24       24              

Impacted Files	Coverage Δ
mcbackend/backends/clickhouse.py	100.00% <100.00%> (ø)

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