Add outline for ST_ShortestPathTree

Unit tests soon to come.
Add unit test outline
Correct weight
Add first unit test

The rest should be relatively easy to implement.
Add wdo2
Add wdo{3,4,5}
Add do tests
Add ro tests
Add wro tests
Add u tests
Add wu tests
Use instanceof ValueDecimal (not ValueDouble)
Add do limited by 1.1 test
Add ro limited by 1.1 test
Add u limited by 1.1 test
Add wdo limited by 6.1 test
Fix expected/actual switchup in unit test
Add wro limited by 6.1 test
Add wu limited by 6.1 test
Add disconnected graph test