LatticeTiling: further simplify XLALComputeMetricOrthoBasis()

- Basis is just the Cholesky decomposition of the inverse of the metric
lalsuite_python.am: replace "exit $?" with "exit 1"

- "exit $?" is redundant, whereas "exit 1" is for super-paranoia
  in case "exec" fails but doesn't return a non-zero exit status