alias return type to workaround cuda error

[nmwsharp]

If this file is included under CUDA 12.8 (and some others), we get an error like:

include/polyscope/scalar_quantity.ipp:365:1: error: no declaration matches ‘std::pair<__remove_cv(double), __remove_cv(double)> polyscope::ScalarQuantity<QuantityT>::getMapRange()’
  365 | std::pair<double, double> ScalarQuantity<QuantityT>::getMapRange() {

This seems to be a buggy interaction between some nvcc and host compiler versions. Aliasing the type works around the error.