UniversalDopplerMetric: print warning instead of error when GSL integrator does not reach requested precision
SuperSkyMetrics: check input metric to XLALSuperSkyMetric() is symmetric and has positive diagonal elements
SuperSkyMetrics: check input metric to XLALReducedSuperSkyMetric() is symmetric and has positive diagonal elements
SuperSkyMetrics: use LU (instead of Cholesky) decomposition to invert frequency-frequency block

- Frequency-frequency block may be not numerically positive definite, but still invertible