Ode

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* Now requires numpy
* Replaces list comprehension addition/multiplication with array math
* update docs/double-pendulum-example to use an array dot-function and
place it on the usage-->examples page of the documentation.
* The date on 0.3.0 was wrong -- should have been 2019-07-20

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* BREAKS COMPATIBILITY
* Output changes from [t, [[x0, v0], [x1, v1] ...]]
to [t, [[x0, x1, ...], [v0, v1, ...]]].

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* Refactor to improve readability and code reuse.
* Clean tempating and testing
* Update documentation

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* Add verlet integration

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* Add generator versions of Euler and backward Euler methods.

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* First release on PyPI.