🐇 Oceananigans v0.2: Abstractions and much faster time stepping

• High-level abstractions for the grid, fields, field sets, constant collections, equation of state, and spectral solver parameters.
• Model code has been refactored so that all operators now modify the fields in place and no memory allocation occurs during time stepping.
• FFTs and DCTs are planned at the start so the quasi-spectral solver can do its work faster.
• Code works in 2D (vertical slices with Nx=1 or Ny=1, and horizontal slices with Nz=1).
• Many more unit tests.
• Examples: isotropic diffusion, horizontal advection, deep convection and Rayleigh-Taylor instability.