Allow request of specific multi-board blocks

[mossblaser]

This has been found to be a potentially useful feature for some folk doing diagnostic stuff.

To implement this, the request() method of the PackTree datastructure would need to be extended to allow the allocation of arbitrary rectangular regions of triads.

Though in principle, allocating arbitrary sets of boards may be useful, in keeping with the rest of the implementation, limiting things to just particular triads makes the most sense.

[alan-stokes]

@lplana does this issue reflect a real issue? or could it be closed due to over complexity? @dkfellows doesn't know what this really means, or more better, he think why is this different from allocating a bunch of specific boards.

[alan-stokes]

@lplana has explained it. valid enhancement which will need to be looked at some point