Fix https://github.com/julianhyde/optiq/issues/200, "Javadoc generation fails under JDK 1.8".

This change fixes a lot of HTML validation issues, and "mvn site" now succeeds under all JDK versions.

"mvn javadoc:javadoc" still has a few errors, because it doesn't use the configuration under "reports" in the POM (it looks for configuration under "build" instead); that is OK.
Add a way to pass a custom parser implementation to Planner.
Oops! Remove bad assert.