PyAT lattices saved in .m format are not recognised by Matlab

[lfarv]

Since numpy 2.0 was released, saving lattice in .m format generate files which cannot be interpreted by Matlab. The reason is that some values are written as "np.float64(1.5)" instead of "1.5" as before. Until this is solved, the recommendation is to use the JSON format, another text format recognised by both Matlab and python