FlatLatticeTiling: return current lattice point through XLALNextLatticePoint()

- by passing optional, pre-allocated gsl_vector pointer
- also fixed a logical bug: tiled_dimensions == 0 block was wrong,
  since parameter space could be made up of single points, and
  main block should handle this case correctly anyway
- passes 'make check'
FlatLatticeTiling: replace XLAL_EFAILED with XLAL_EFUNC/XLAL_EINVAL as appropriate