Fix check marks in Manual
[Doxygen] use \internal
[Doxygen] Move Debugging module to core
[Doxygen] Collect Attribute docs into Core/Attributes
[Doxygen] Organize Core/Random Variables
[Doxygen] src/core
Fix unit conversion of negative Times

Thanks to Vedran for finding this.

Example:

  Time tneg = Seconds (-1);
  NS_LOG_UNCOND ( tneg << ", in s: " << tneg.ToInteger (Time::S));

produces

  -1000000000.0ns, in s: 18446744072

Correct result is

  -1000000000.0ns, in s: -1
Change upper-bounded TimeChecker to actually do what documentation says