add isl_options_{get,set}_pip_symmetry

add isl_options_{get,set}_pip_symmetry

This allows users of the isl library to turn off symmetry detection.
Symmetry detection can result in a significant speed-up on some inputs.
For example, on testsets/pip/cnt_sum2.pip, isl_pip takes 0m20.656s
without symmetry detection and 0m1.540s with symmetry detection.
The representation of the result is also much smaller with symmetry detection.

However, symmetry detection introduces an extra parameter, which
may result in some computations taking much longer, even to the point
of offsetting the advantages of symmetry detection.
For example, for the binary relation

{ S_2[i, k, j] -> [o0, o1, o2, o3] : exists (e0 = floor((o1)/8):
	8e0 = o1 and k <= 255 and o0 <= 255 and o2 <= 255 and
	o3 <= 255 - j and 7o3 >= 255 - j and 7o3 <= 7 - i and
	240o3 <= 239 + o0 and 247o3 <= 247 + k - j and
	247o3 <= 247 + k - o1 and 247o3 <= 247 + i and
	248o3 >= 248 - o1 and 248o3 <= o2 and 254o3 >= i - o0 + o1
	and 254o3 >= -o0 + o1 and 255o3 >= -i + o0 - o1 and
	1792o3 >= -63736 + 257o1) }

reported by Wenlei Bao, lexicographic minimization takes
7m24.836s with symmetry detection and 0m0.304s without.
In particular, the minimization procedure seems to be taking
a lot more time exploring parts of the context based purely
on the parameters added by the symmetry detection,
introducing many extra integer divisions.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>