[dive] simplify and generalize the computation of values & range rating

headers/header_spec.txt

ivette/src/renderer/Dive.tsx

ivette/src/renderer/dive_style.js

src/plugins/dive/Makefile.in

src/plugins/dive/build.ml

src/plugins/dive/dive_graph.ml

src/plugins/dive/dive_graph.mli

src/plugins/dive/dive_types.mli

src/plugins/dive/node_range.ml

src/plugins/dive/node_range.mli

src/plugins/dive/server_interface.ml

src/plugins/dive/tests/dive/oracle/ranges.dot

src/plugins/dive/tests/dive/oracle/ranges.res.oracle

src/plugins/dive/tests/dive/oracle/various.dot

src/plugins/dive/tests/dive/ranges.i