Offsetting kernel indices

[glwagner]

It'd be extremely useful to have a feature for offsetting the kernel index, either at launch time or setup time.

Not sure what the best design or interface would be. @simone-silvestri and I discussed possibly embellishing StaticSize:

KernelAbstractions.jl/src/nditeration.jl

Lines 14 to 17 in 4128c3d

	struct StaticSize{S} <: _Size
	function StaticSize{S}() where S
	new{S::Tuple{Vararg{Int}}}()
	end

Our work exclusively uses statically sized kernels so I think we'd prefer a way to specify the offsets statically; eg the size and offset are simultaneously determined.

But another possibility would be to accept offsets as a kwarg similar to how ndrange is accepted for DynamicSize?

[vchuravy]

I talked with Simone and I think expressing this as an Index projection similar to LinearIndicies and CartesianIndices is the right direction.

[b-fg]

In WaterLily we could also benefit from this as we are currently passing index offsets manually too.