Provide TreeSequence array access to ragged columns

[jeromekelleher]

#2424 added direct numpy access to the non-ragged table columns from the TreeSequence object. We should provide access also for the ragged columns (at some point). Some issues:

1. Should these be direct arrays so we just reflect the original data and offset arrays or should they be decoded? (arguments both ways)
2. We need to check that the metadata arrays are guaranteed to exist (do we allow creating a ts from tables with NO_EDGE_METADATA)?

[petrelharp]

I vote for decoded?

[jeromekelleher]

Yeah, decoded seems most useful, I agree.