Provide stable download location for dawn and dawncut

The http://geant4.kek.jp/ location is down often enough that this is a productivity issue. At this point we are not able to shift from dawn to other visualization solutions, though.

### Description

http://geant4.kek.jp/ is currently down. The dawn source is at http://geant4.kek.jp/~tanaka/src/dawn_3_91a.tgz. The dawncut source is at http://geant4.kek.jp/~tanaka/src/dawncut_1_54a.taz.