ColumnStats on Append and Timestamp handling

[osopardo1]

Description

This PR fixes bugs #204 and #195

Type of change

It is a bug fix for Timestamp and Column Stats handling. Both issues were touching the same code, so I find more useful to solve them both together.

1. Issue Appending data with new columnStats fails in some cases #195 was due to an inconsistent creation of Revisions when appending new data. The merge of Revisions was not done correctly if the user specify a wider region of the space.
2. Issue Column stats for Timestamp are not saved properly #204 was due to bad parsing of JSON Timestamps.

Checklist:

Here is the list of things you should do before submitting this pull request:

• 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? (Optional)

This is tested with two different tests in SparkRevisionFactoryTest.

Test Configuration:

• Spark Version: 3.3.x
• Hadoop Version: 3.3.4
• Cluster or local? Local

[codecov]

Codecov Report

Merging #205 (1f67422) into main (35bc1f0) will decrease coverage by 0.12%.
The diff coverage is 94.44%.

❗ Current head 1f67422 differs from pull request most recent head 72ae325. Consider uploading reports for the commit 72ae325 to get more accurate results

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
- Coverage   93.87%   93.76%   -0.12%     
==========================================
  Files          85       85              
  Lines        2091     2100       +9     
  Branches      172      173       +1     
==========================================
+ Hits         1963     1969       +6     
- Misses        128      131       +3     

Files Changed	Coverage Δ
...ain/scala/io/qbeast/spark/table/IndexedTable.scala	94.33% <93.33%> (-0.61%)	⬇️
...scala/io/qbeast/spark/internal/QbeastOptions.scala	96.42% <100.00%> (+0.27%)	⬆️

... and 1 file with indirect coverage changes

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