Fix for issue #911 (backport #926)

[mergify]

Summary

Internally the subclass of trainable kernel would call its super init where BaseKernel would create a default feature_map, if none was passed, so that the feature_map attribute afterwards was a valid feature_map. Now the derived classes, in places used feature_map instead of self.feature_map where the former was still None. This changes the code to use self.feature_map and added tests to ensure construction is successful.

closes #911

Details and comments

---

This is an automatic backport of pull request #926 done by [Mergify](https://mergify.com).

[coveralls]

Pull Request Test Coverage Report for Build 15194876158

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.637%

---

Totals
Change from base Build 15194846110:	0.0%
Covered Lines:	4366
Relevant Lines:	4817

---

💛 - Coveralls