Deprecate is_spline_interpolation_mesh_uniform

[EmilyBourne]

The function is_spline_interpolation_mesh_uniform does not make sense. It is described as:

ddc/include/ddc/kernels/splines/spline_builder.hpp

Lines 24 to 35 in b8ccaf4

	/**
	* @brief A helper giving the uniform/non_uniform status of a spline interpolation mesh according to its attributes.
	*
	* A helper giving the uniform/non_uniform status of a spline interpolation mesh according to its attributes.
	*
	* @param is_uniform A boolean giving the presumed status before considering boundary conditions.
	* @param BcXmin The lower boundary condition.
	* @param BcXmax The upper boundary condition.
	* @param degree The degree of the spline.
	*
	* @return A boolean giving the uniform/non_uniform status.
	*/

However in reality it gives the uniform/non_uniform status of greville abcissae. The interpolation points have been decoupled from the greville abcissae for a while so this function should not be used. It should therefore be deprecated

[EmilyBourne]

This was discussed in https://github.com/CExA-project/ddc/pull/409/files#r1579627086

Following https://gitlab.maisondelasimulation.fr/gysela-developpers/gyselalibxx/-/merge_requests/511 this function is no longer used in Gysela

[tpadioleau]

Ok if it is no longer used then we will bypass the deprecating stage and remove it.