Add linear tree classifier

[Sinacam]

What does this PR do?

Adds tree classifier and modifies linear.py, main.py and linear_trainer.py to accommodate it.
This is a breaking change for loading previously stored linear pipelines.

Test CLI & API (bash tests/autotest.sh)

• Test Pass
  • (Copy and paste the last outputted line here.)
• Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_examples.sh)

• Test Pass
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• Not Applicable (i.e., the PR does not include API changes.)

Check API Document

• API document is updated (linear, nn)
• Not Applicable (i.e., the PR does not include API changes.)