Euler computations

[masa10-f]

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Description of the change:

This PR implements Euler computations required in the library. The detail is described in the euler.py

[shinich1]

minor comments

[masa10-f]

@shinich1 Thank you for your review! I have fixed them in 0c4dda9

[EarlMilktea]

@masa10-f Please check fbcbc18 (euler-ss branch).