Precision resilient 2D Laplacian

[milankl]

ShallowWaters.jl/src/Gradients.jl

Line 37 in 5b5746b

du[j,i] = minus_4*u[j+1,i+1] + u[j,i+1] + u[j+2,i+1] + u[j+1,i] + u[j+1,i+2]

probably as

uij = u[j,i]
∇²u[j,i] = (u[j,i+1]-uij) + (u[j+2,i+1]-uij) + (u[j+1,i]-uij) + (u[j+1,i+2]-uij)

[milankl]

or

∇²u[j,i] = ((u[j-1,i]-uij) + (u[j+1,i]-uij)) + ((u[j,i-1]-uij) + (u[j,i+1]-uij))

note the brackets

[milankl]

done in #153