Fix decoration for atan with two arguments

[OlivierHnt]

This PR changes the decoration returned by atan(y, x) whenever inf(y) >= 0 and y contains 0 and sup(x) <= 0.

N.B.: atan is a discontinuous function at the half-line y = 0, x < 0, which is included in its domain.

Before this PR

julia> atan(interval(0), interval(-2, -1))
[3.14159, 3.1416]_dac

which I believe is wrong since both input intervals and output intervals are bounded.

So, with this PR

julia> atan(interval(0), interval(-2, -1))
[3.14159, 3.1416]_com

Moreover, there is a test in the ITF1788 which actually depends on this choice of decoration:

@test atan(interval(bareinterval(-0.0, 1.0), com), interval(bareinterval(-2.0, -0.1), com)) === interval(bareinterval(0x1.ABA397C7259DDP+0, 0x1.921FB54442D19P+1), dac)

which I think is wrong (for me the decoration should be com and not dac). I changed this test to

@warn "The original test `atan2 [-0.0, 1.0]_com [-2.0, -0.1]_com = [0X1.ABA397C7259DDP+0, 0X1.921FB54442D19P+1]_dac` is wrong and has been modified. The result should have the decoration `com`"
@test atan(interval(bareinterval(-0.0, 1.0), com), interval(bareinterval(-2.0, -0.1), com)) === interval(bareinterval(0x1.ABA397C7259DDP+0, 0x1.921FB54442D19P+1), com)