Fix Scene not being copied properly during resampling

[djhoese]

This PR addresses the issues brought up in #208 as well as a couple refactors.

1. Fix Scene wishlist being shared between a scene and the new scene generated during resampling.
2. Fix Scene dependency tree being shared between scenes after resampling.
3. Fix loading not being able to handle dependency tree nodes created by user's manually adding datasets to the Scene.
4. Fix use of deprecated self.assertEquals in Scene tests.
5. Rename _read_datasets and _read_composites in Scene since they require internal objects to be passed to them and should not be accessed by users.

• Closes Strange behaviour when trying to load data to a scene object after having worked with it #208
• Tests added
• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[mraspaud]

Please write "This should be impossible" as a comment.

[coveralls]

Coverage increased (+0.4%) to 59.49% when pulling 7b08c06 on bugfix-scene-copy into 56d2b6a on develop.

[coveralls]

Coverage increased (+0.4%) to 59.49% when pulling 82c12af on bugfix-scene-copy into 56d2b6a on develop.