Additional changes for WG LC preparation

[italobusi]

Changed name for gsnr-margin: fix #87

Made line-coding-bitrate configurable: fix #85

Updated l0-tunnel-attributes grouping: fix #84

• removed bit-stuffing attribute from l0-tunnel-attribute grouping
• removed term-type base identity and all its derived identities

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Co-authored-by: sergio belotti sergio.belotti@nokia.com