FP Precision

New features:

• Consistent floating point precision using math.set_precision
• MacCormack advection scheme
• Advanced demo scripts: rotating_bar.py, vortex_fit.py
• Basic implementation of Broyden's method for root finding
• Lots of convenience updates: CenteredGrid.sample(function), Domain.dx, general_grid_sample_nd, interpolate

Breaking changes:

• Finite difference in gradient now uses central difference by default
• Noise now only has a single channel unless otherwise specified

Pull request #18

This is the last major update of the Φ_Flow 1.x series.