Real-Time Fluid Dynamics for Games, Jos Stam 2003 [JS port]

This is a direct port from the C code presented in Stam's 2003 paper, with a small convenience wrapper.

Example:
``js
const {Fluid} = require('stam-stable-fluids')
const size = 128
const f = new Fluid(size)
for (let y = 0; y < size; y++)
for (let x = 0; x < size/2; x++)
f.addDensity(x, y, 10)
for (let y = 0; y < size; y++)
f.addForce(10, y, 10, 0)
f.step(1/60)

const [vx, vy] = f.velocityAt(0, 0)
const d = f.densityAt(0, 0)
`

API

#### new Fluid(size, viscosity = 0, diffusion = 0)

Make a new fluid simulation with width and height equal to size. If you want a goopier fluid, like honey, set viscosity to a small positive number, like 0.0001 (larger numbers will probably make the fluid so "thick" it will barely move). diffusion will make the "stuff" in the fluid spread out over time. The default for both viscosity and diffusion, if not provided, is 0.

#### Fluid.step(dt)

Step the simulation by dt (e.g. 1/60).

#### Fluid.velocityAt(x, y)

Returns [vx, vy] representing the velocity of the fluid at (_x_, _y_).

#### Fluid.densityAt(x, y)

Returns the density of "stuff" at (_x_, _y_).

#### Fluid.addForce(x, y, fx, fy)

Adds an external force to the fluid, e.g. from a UI or other system. The force field will be cleared after every step().

#### Fluid.addDensity(x, y, d)

Adds an external density flow to the fluid. d is the rate of flow. The density flow field will be cleared after every step()`.