Add helper functions to create uniform NonUniformSampling and NonUniformBSplines

Tests on `NonUniformBSplines` often use uniform break points. We could add a helper to generate the break points to be able to write something like:
```cpp
struct X {};
using NUBSplinesX = ddc::NonUniformBSplines<X, 3>;
ddc::Coordinate<X> const rmin(0.);
ddc::Coordinate<X> const rmax(1.);
std::size_t const ncells = 10;
ddc::ini_discrete_space<NUBSplinesX>(rmin ,rmax, ncells);
```