Made Adapters Sliceable

[eodole]

I made the adapters sliceable but one concern I have is that when I ask for generic_adapter[0:2] this will return an entirely new adapter instance. Depending on the use case this one matter but could cause problems. I also made a test file but it's not tracked by github, please let me know if I should also upload that.

I also noticed that ElementwiseTransform wasn't being imported so added that as well.

I thought it might also be nice to add the indices of the transforms to the default adapter print, but didn't add that particular feature. I also thought it might be nice to add the feature of appending a new transform via slice assignment so if index == len(adapter.transforms) then simply use the add_transform method.
Resolves #274

[paul-buerkner]

Thank you! I will take a look soon!

[paul-buerkner]

A few short questions:

• Where do you anticipate issues with slicing creating new adapter instances? To me it sounds intuitive that we would create a new adapter instance.
• A test file (if set up in the appropriate test folder etc.) would be great and should be added to this commit I think.
• Index numbers in the printing would be great! How much effort would it be for you to add this?
• I don't fully understand what you mean by "appending a new transform via slice assignment so if index == len(adapter.transforms) then simply use the add_transform method." Can you elaborate? Also perhaps use some example code how what you have in mind would look?