X-ray absorption spectroscopy, EELS, and full-potential augmented plane wave study of the electronic structure of\({\mathrm{Ti}}_{2}\mathrm{Al}\mathrm{C}\),\({\mathrm{Ti}}_{2}\mathrm{Al}\mathrm{N}\),$ {\mathrm{Nb}}_{2}\mathrm{Al}\mathrm{C}$, and\({\left({\mathrm{Ti}}_{0.5}{\mathrm{Nb}}_{0.5}\right)}_{2}\mathrm{Al}\mathrm{C}\)