RFR: 6948: Add union to QuantityRange

[Jean-Philippe Bempel]

Add tests for intersetion & union

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Commit messages:
 - 6948: Add union to QuantityRange

Changes: https://git.openjdk.java.net/jmc/pull/150/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=150&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6948
  Stats: 112 lines in 2 files changed: 112 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/150.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/150/head:pull/150

PR: https://git.openjdk.java.net/jmc/pull/150