Add numpy arrays to Python Tree object

[jeromekelleher]

It will be useful for some applications to have direct access to the underling parent, left_child etc arrays in the quintuply linked tree representation. We can do this by adding some (read only) numpy arrays that use these pointers defined in the C structs, i.e.,

typedef struct {
    PyObject_HEAD
    TreeSequence *tree_sequence;
    tsk_tree_t *tree;
    PyObject *left_array;
    PyObject *right_array; // etc
} Tree;

Then in Tree_init we can create the required numpy views, and make them available to Python following the steps here

[jeromekelleher]

See tutorial suggestion for an idea of how we might test out how useful this is.

[jeromekelleher]

After a fun afternoon playing with numba (see the tutorial thread above), this has gone a long way up my priority list! I'd kinda like to get it in for 0.3.6 @benjeffery and I'm happy to do the coding (I'm confident it's straighforward, and it'd be a nice break from other stuff). What's the timeline for 0.3.6?

[benjeffery]

Awesome stuff, numba is excellent. Looking at the 0.3.6 milestone I would hope to have it complete within 2 weeks, hopefully before. Happy to add this issue to that milestone.