add chemv example

add chemv example

ppcg used to take an extremely long time on this example.
In particular, the isl_set_dim_max calls in extract_size
(now in ppcg_size_from_extent called from extract_grid_size)
would take a very long time.
A workaround was to compute explicit representations for
the existentially quantified variables using isl_set_compute_divs
prior to calling isl_set_dim_max, but this would result
in more complicated expressions on other examples.

Since dd37a9c (gpu: extract effective grid size from schedule tree,
Fri Oct 4 14:23:53 2013 +0200), the representation of the extent
of the grid is much simpler and the above problem no longer exists
for this example.

Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>