Update IForest model

[FrizzoDavide]

All Submissions Basics:

• [OK ] Have you followed the guidelines in our Contributing document?
• [ OK] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ OK] Have you checked all Issues to tie the PR to a specific one?

All Submissions Cores:

• [ OK] Have you added an explanation of what your changes do and why you'd like us to include them?
  Added some properties to the IForest class so that new methods for IForest implementing the DIFFI algorithm, that will be inserted in a future Pull Request, can work also on objects of the IForest class
• [OK ] Have you written new tests for your core changes, as applicable?
• [OK ] Have you successfully ran tests with your changes locally?
• Does your submission pass tests, including CircleCI, Travis CI, and AppVeyor?
• [OK ] Does your submission have appropriate code coverage? The cutoff threshold is 95% by Coversall.

[coveralls]

Pull Request Test Coverage Report for Build 6748166519

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 93.726%

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Totals
Change from base Build 6637187637:	0.008%
Covered Lines:	9023
Relevant Lines:	9627

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💛 - Coveralls