🥧 Oceananigans v0.3: More model features and super hacky GPU support.

Features

• Main struct is now the Model instead of the Problem. It should be neater too.
• Support for no-slip boundary conditions at the top and bottom.
• Boundary condition information stored in a BoundaryConditions struct.
• Abstract output writer (checkpoints and writing fields to disk) and diagnostics framework.
• ModelMetadata struct to store architecture (e.g. :cpu or :gpu) and floating point number precision.
• ModelConfiguration struct to store physical parameters such as viscosity and diffusivity.
• Clock struct keeps track of time, time step number, and Δt.
• Refactored δ! and avg! operators to use @views @. so they work on CPUs and GPUs.
• GPU-compatible DFT! and IDFT! operators based on FFT! and IFFT! operations.
• Super hacky and very slow GPU support (but it works out of the box!). This will be overhauled ASAP.
• Support for 1D vertical models.
• More examples: stratified deep convection, Rayleigh–Bénard convection, 1D vertical.

Main bug fixes

• Added in Coriolis term for u and v-momentum equations (important for 3D simulations). ali-ramadhan@1b07df1
• Fixed typo in u∇w! operator causing blowup in some 2D simulations (Ay and Az were mixed up). ali-ramadhan@d7326c8