fix adaptivesampling! to only split leaf cells

[cserteGT3]

Subsequent calls of adaptivesampling! fail because it just collects the cells and splits them if required. MWE from example:

using RegionTrees
using StaticArrays: SVector
import RegionTrees: AbstractRefinery, needs_refinement, refine_data

struct MyRefinery <: AbstractRefinery
    tolerance::Float64
end

function needs_refinement(r::MyRefinery, cell)
    maximum(cell.boundary.widths) > r.tolerance
end

function refine_data(r::MyRefinery, cell::Cell, indices)
    boundary = child_boundary(cell, indices)
    "child with widths: $(boundary.widths)"
end

r = MyRefinery(0.05)
root = Cell(SVector(0., 0), SVector(1., 1), "root")

Then calling adaptivesampling! twice:

julia> root = Cell(SVector(0., 0), SVector(1., 1), "root")
Cell: HyperRectangle{2, Float64}([0.0, 0.0], [1.0, 1.0])

julia> adaptivesampling!(root, r)
Cell: HyperRectangle{2, Float64}([0.0, 0.0], [1.0, 1.0])

julia> adaptivesampling!(root, r)
ERROR: AssertionError: isleaf(cell)
Stacktrace:
 [1] macro expansion
   @ C:\Users\user\.julia\packages\RegionTrees\rosFd\src\cell.jl:76 [inlined]
 [2] split!(cell::Cell{String, 2, Float64, 4}, child_data::RegionTrees.TwosArray{2, String, 4})
   @ RegionTrees C:\Users\user\.julia\packages\RegionTrees\rosFd\src\cell.jl:65
 [3] split!
   @ C:\Users\user\.julia\packages\RegionTrees\rosFd\src\cell.jl:69 [inlined]
 [4] adaptivesampling!(root::Cell{String, 2, Float64, 4}, refinery::MyRefinery)
   @ RegionTrees C:\Users\user\.julia\packages\RegionTrees\rosFd\src\adaptivesampling.jl:13
 [5] top-level scope
   @ REPL[11]:1

This PR fixes this by only splitting leaf cells and adding the children of others instead so subsequent calls work as intended.

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[cserteGT3]

@rdeits If you have time, could you please take a look at my PRs? I hope them to be easy to review.