Add flags_array and time_array to Tree class?

[jeromekelleher]

#1320 added arrays for the underlying tree vectors. I think it's probably a good idea to add the flags_array and time_array to the Tree class also, since these are things we'll often be using in low-level algorithms, and it's a faff (and currently copy-ful, semantics-wise) to do this via the tree sequence tables arrays.

On a technical node, I think we'd probably need to do this from the bottom up like the Tree arrays, using the Tree class as the base object for the numpy arrays. We could just create arrays in the high-level Tree class, but these would be copies and so technically incur a perf penalty (even if it would probably be negligible).

[benjeffery]

Bumping this to 0.3.7

[jeromekelleher]

Bumping this up the priority list as we need it for phylokit work.

[jeromekelleher]

A tricky issue here is how we deal with the fact that the dimensions of these arrays are different to the other tree arrays. If we use a direct pointer to the table memory, then we don't have any space for the time of the virtual root.