Adding support for indexing Date data types

[Adricu8]

Description

Adding support for indexing Datetime data type in qbeast-spark.
In reference to issue.

Type of change

It is an enhancement that would allow for Datetime data type to be indexed without the need of a workaround like casting the value to a Long type or similar.
Spark has two date data types, we could add support for both.

More elaborated explanation and guidelines regarding the addition of new types here

Checklist:

• New feature / bug fix has been committed following the Contribution guide.
• Add comments to the code (make it easier for the community!).
• Change the documentation.
• Add tests.
• Your branch is updated to the main branch (dependent changes have been merged).

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• TransformerTest: I have added a test to check the correct behavior of LinearTransformation with Timestamps
• TransformerIndexingTest: I have added a test to ensure timestamps read and write correctly with spark

Test Configuration:

• Spark Version: 3.1.1
• Hadoop Version: 3.2
• Running Locally on M1

[codecov]

Codecov Report

Merging #129 (2f27554) into main (cc3df2c) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 2f27554 differs from pull request most recent head 68ed1ba. Consider uploading reports for the commit 68ed1ba to get more accurate results

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   91.68%   91.77%   +0.08%     
==========================================
  Files          62       62              
  Lines        1432     1446      +14     
  Branches      106      108       +2     
==========================================
+ Hits         1313     1327      +14     
  Misses        119      119              

Impacted Files	Coverage Δ
...rc/main/scala/io/qbeast/core/model/QDataType.scala	100.00% <100.00%> (ø)
...o/qbeast/core/transform/LinearTransformation.scala	89.33% <100.00%> (+0.60%)	⬆️
...a/io/qbeast/core/transform/LinearTransformer.scala	100.00% <100.00%> (ø)
...cala/io/qbeast/core/transform/Transformation.scala	66.66% <100.00%> (+16.66%)	⬆️
...ala/io/qbeast/spark/utils/SparkToQTypesUtils.scala	92.30% <100.00%> (+1.39%)	⬆️

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[osopardo1]

Well done! Just a few more comments/questions:

1. We could put something more user-friendly in the title of the PR. For example: Support for indexing Date columns. Then everyone can understand what this is about. Because Transformer and QDataTypes are something more dependent on implementation rather than functionality. 👨‍💻
2. I guess for the implementation we are only adding Timestamp. But you have intentions to add a Date as well, right?

[osopardo1]

Looks good!