graph/layout: add Isomap layout
ADD
graph/layout/testdata/line_isomap_golden.png
graph/layout/testdata/line_isomap_golden.png
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graph/layout/testdata/sheet_isomap_golden.png
graph/layout/testdata/sheet_isomap_golden.png
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graph/layout/testdata/square_isomap_golden.png
graph/layout/testdata/square_isomap_golden.png
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graph/layout/testdata/tetrahedron_isomap_golden.png
graph/layout/testdata/tetrahedron_isomap_golden.png
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graph/layout/testdata/tube_isomap_golden.png
graph/layout/testdata/tube_isomap_golden.png
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graph/layout/testdata/wp_page_isomap_golden.png
graph/layout/testdata/wp_page_isomap_golden.png
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graph/layout: add Isomap layout