Adopting new identities for wavelength assignment mechanism

[aguoietf]

In the YANG model for WDM tunnel there are two new defined identities for wavelength assignment mechanism:

  identity lower-first-wavelength-assignment {
    base l0-types:wavelength-assignment;
    description
      "Allocate wavelengths in ascending order, beginning from the
       lowest frequency and progressing toward the highest frequency
       within the permissible frequency range.";
  }

  identity upper-first-wavelength-assignment {
    base l0-types:wavelength-assignment;
    description
      "Allocate wavelengths in decending order, beginning from the
       highest frequency and progressing toward the lowest frequency
       within the permissible frequency range.";
  }

Since these new identities are generic and are intended to be shared by other models such as optical path computation, my proposal is to move them to l0-types.