Use += to add to EXTRA_DIST
LatticeTiling: documentation edits
LatticeTiling: add more descriptive error messages to GCALL() calls

- Except gsl_linalg_QR_unpack(), which doesn't need error checking
LatticeTiling: compute bounding box later in XLALSetLatticeTypeAndMetric()
LatticeTiling: use Cholesky decomposition in XLALComputeMetricOrthoBasis()

- Simpler implementation that Gram-Schmidt, gives similar numerical results
- Can remove positive definiteness check in XLALSetLatticeTypeAndMetric()