Add support for extended slicing in Leaf objects

[faltet]

For example, given this:

rows = [
    ('foo', 0.0, 0.0, 150.0),
    ('bar', 0.5, 0.0, 100.0),
    ('foo', 1.0, 1.0,  25.0)
]
rownos = [2, 735, 371913476]

providing support for extended slice would allow to write this:

tbl[rownos] = rows

instead of:

for row_id, datum in zip(rownos, rows):
     tbl[row_id] = datum

Also, this support can leverage the sparse read/write capability of HDF5.

[GabrielBeckers]

Just a note to let you know that this feature would be very useful to me. I run into a lot of situations where I would like to use a list as an index for an (E)Array, e.g. ar[:10,[2,5,7]]. The example in the ticket is about a Table, but I am hoping that Arrays will be included.

Cheers, Gabriel