* addition of test for equality checking of ODE Ops
(not yet implemented)

* WIP: refactoring the DifferentialEquation Op
+ full support for test_values
+ explicit input/output types
+ 2D return shape
+ optional return of sensitivities
+ gradient without helper Op

* fully replace DifferentialEquation Op with the refactored implementation

* align tests with refactored API
+ whitespace & condensed formatting
+ test for equality of identical Ops

* use tt.stack as suggested by DeprecationWarning

* always cast y0 and theta to floatX

* allow some tests to fail on float32
(due to downcast exception)

* don't use f-strings to maintain 3.5 support

* link ODE refactor PR

* renamed ODE notebooks
+ add notebooks to examples index

* use (custom) errors instead of asserts

* move ShapeError to exceptions.py