Scala 2.13: Second take

[amivanoff]

Let's enter 2025 with Scala 2.13 support! 😊 Happy new year! 🎄🎆🎄🎉🎄🎁🎄

Second take on Scala 2.13, based on unfinished PR #860. Related to #811.

1. Some sort of hand-made cross-compilation inside:

Scala 2.13: compiles and passes all tests (as far as I understand).

• But many warnings left in code to maintain compatibility with 2.12
• And also it blows IntelliJ Idea's mind: it detects some strange errors

Scala 2.12: compiles and passes all tests (as far as I understand).

• But in requires to comment import scala.collection.parallel.CollectionConverters._ in OneVsRest.scala.

2. Due to immutable Seq in transformers in 2.13, there was a problem with MinMaxScalerPipelineParitySpec test -- Array values in ArrayRows should be immutable. I made some fix in TypeConverters.scala to ensure immutability. Not sure if it is the right place.

3. No automatic cross-compilation in Travis due to 2. Don't know what is the best practice to handle cross-compilation in the Scala world.

There are at least two variants:

• Conditional Compilation Some hacks, no official way
  • We need hardcore low level ifdefs "before parser" preprocessor which works with TEXT LINES inclusion/exclusion (imports lines, etc,). Not high-level "type annotations" and types inclusion/exclusion.
• Separate branches -- it seems it's an overkill
• Separate folders -- also too clunky