Add a `basis_domain` method to Bsplines

[EmilyBourne]

Bsplines have 1 method which returns the "full domain":

ddc/include/ddc/kernels/splines/bsplines_uniform.hpp

Lines 312 to 319 in bb0f547

	/** @brief Returns the discrete domain including eventual additional B-splines in the periodic case. See size().
	*
	* @return The discrete domain including eventual additional B-splines.
	*/
	KOKKOS_INLINE_FUNCTION discrete_domain_type full_domain() const
	{
	return discrete_domain_type(m_reference, discrete_vector_type(size()));
	}

however it is common to require the domain without duplicate bsplines. This can be obtained with:

ddc::discrete_space<BSplinesR>().full_domain().take_first(ddc::DiscreteVector<BSplinesR>(ddc::discrete_space<BSplinesR>().nbasis()));

but this is very wordy for such a simple calculation. It would make sense to provide this functionality

[tpadioleau]

Sure looks ok to me.