MNT: Bump ciftilib to v1.6.0.
Bump libxml++ to 4.0, as it is now available on conda-forge. Corresponding changes to ciftilb sources (libxml++ API changes) and cmakefiles (new ABI versions for libxml++, libxsigcpp and glibmm)
Bump libxml++ to 4.0, as it is now available on conda-forge. Corresponding changes to ciftilb sources (libxml++ API changes) and cmakefiles (new ABI versions for libxml++, libxsigcpp and glibmm)