UnitCell.GetLatticePar() without index fails on nanobind_migration_test #127

from pyobjcryst.crystal import Crystal

c = Crystal(4.76, 4.76, 12.99, "R-3c")

print(c.GetLatticePar(0))  # 4.76
print(c.GetLatticePar())   # main: array of 6 values; nanobind branch: TypeError
# TypeError: Unable to convert function return value to a Python type!
#     GetLatticePar(self) -> CrystVector<double>

On main, GetLatticePar() returns numpy array of 6 lattice params through the globally used CrystVector_REAL_to_ndarray converter.

On the nanobind branch, nb_unitcell.cpp:95 binds the overload directly, and since there's no converter for CrystVector<double>, the conversion fails. Claude suggests wrapping it in crystvec_to_array to fix it.

All existing tests call GetLatticePar(index) so didn't catch this regression. Other methods returning CrystVector might have the same problem.

(tested on 3452813)