[RFC][oneMKL] Rename oneMKL Interface project

[Rbiessy]

This RFC is linked to the main RFC proposed in uxlfoundation/oneMath#564.

The proposition is to rename and move the oneMKL Interface to avoid confusion with the Intel oneMKL product. The suggested new name is oneMath for now.

The implications on the specification are:

• Moving the folder source/elements/oneMKL to source/elements/oneMath
• Renaming the files using "mkl" or "onemkl" to "onemath"
• Renaming the occurrences of "oneMKL" and "onemkl" to "oneMath" and "onemath" respectively
• Renaming the namespace oneapi::mkl to oneapi::math and references to the folder oneapi/mkl to oneapi/math
• Renaming remaining occurrences of "mkl" to "onemath" (internal variables and references to filenames)