YANG Doctor LC comments (#70)

YANG module updates to fix YD last call comments: fix #68

Updated FEC identities: fix #16

Removed DC-DP-QAM8 and DC-DP-QAM16 identities: fix #7

Changed names of power-in-db data types into ratio-in-db: fix #74 

Added support for multiple line codings for the same ITU-T application code or for the same organizational mode: fix #62 

Added the base identity for operational-mode definitions: fix #9 

Clarified how available nominal central frequencies in flexi-grid are advertised: fix #61

Updated reference to RFC8776-bis: fix #71

Co-authored-by: sergio belotti <[email protected]>