Add an Euler model (#196)

* Add Euler module

* Add simple vortex example

* Document Euler operator and add pulse experiment

* Test euler examples in lazy mode

* Make rk4_step importable

* Remove copypasta from example headers

* Make vortex initial condition importable

* Add Euler convergence test to test suite

* Rename: EulerContainer -> EulerField

* Clean up vortex initial condition

* Add type annotations and rename array container

* Clean up helper functions

* Add Euler module to the docs