[Persistence Interval] fix of structural binding bug in windows compiler

[hschreiber]

Just moved the get from the Gudhi::persistence_matrix namespace to a member method, as Windows confuses them with the ones in the std namespace otherwise. The git diff got very confused by the changes, the best is to probably just look directly at the file.

[mglisse]

The git diff got very confused by the changes

This version of the diff is rather readable.

[mglisse]

An alternative would be: do not specialize tuple_size, get rid of the unnecessary get members, reorder the members of the class, and rely on "case 3" of structured binding. But this version is good.