Integration with Chmy.jl

[boriskaus]

Chmy.jl is a package that can be used to write (staggered) finite difference codes that run on CPU and (parallel) GPU's.
This PR integrates the GeophysicalModelGenerator package with Chmy such that we can use all functionality of GMG to create initial model setups.

Example:

using Chmy, Chmy.Architectures, Chmy.Grids, Chmy.Fields
using KernelAbstractions # for backend-agnostic kernels

backend = CPU()
arch = Arch(backend)

# 3D Chmy grid
lx, ly, lz  = 10.0, 11.0, 12.0
nx, ny, nz  = 10,11,12
grid   = UniformGrid(arch;
                    origin=(-lx/2, -ly/2, -lz/2),
                    extent=(lx, ly, lz),
                    dims=(nx, ny, nz))

# Create Chmy fields
Temp_C   =  Field(backend, grid, Center())
Phases_C =  Field(backend, grid, Center(), Int32)

# Add a box
add_box!(Phases_V,Temp_V,grid, xlim=(0,1.0), zlim=(-2,0), phase=ConstantPhase(2))