Release v1.4.0

[darothen]

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