Release v1.4.0

A litany of things to do in prep for a nice, new v1.4.0 release:

Update documentation to include more references to work using this paper
Deprecate all Cython for Numba
(potentially) re-factor integration solvers to use a different ODE solver library (either call out to Julia's DifferentialEquations or possibly scikit-odes
Re-factor internal arrays to wrap behind xarray
Create new Factory-style initialization procedure for easier customizability of the core simulation
black formatting
More complete type annotations throughout code
Deprecate Python <= 3.5 support
Move from TravisCI to CircleCI

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