GammaLib

The solid angle in GSkyRegionRectangle::compute_solid_angle is currently computed using

m_solid = m_width * m_height * gammalib::deg2rad * gammalib::deg2rad;

which is not correct on the sphere. The correct formula is

m_solid = m_width * gammalib::deg2rad * 2.0 * sin(0.5 * m_height * gammalib::deg2rad);