FourierFlows/FourierFlows.jl: FourierFlows v0.3.0

This release introduces some major code refactors. In particular, AbstractType Clock replaces AbstractType State (see PR #96). Some additional changes include: Allows for general sol, i.e., an array that consist of arrays of different size and type. Generalizes time-steppers: no need for "dual" time-steppers. Moves all physics modules (except Difffusion) into either GeophysicalFlows.jl or PassiveTracer.jl. Changes Diagnostic.step --> Diagnostic.i. Removes 2D wavenumber grids K, Kr, and L from TwoDGrid module