update to fail on duplicate LN tags, with different LN values

[vasudeva8]

Fixes #1866
Updated to use 1st LN tag value as in cram handling.
Accepts duplicate LN tags as long as they have same value.
If the values differ, returns failure.

sam_hdr_create updated for sam file checks and sam_hdr_sanitise for handling bam and cram.
sam_hdr_sanitise calls sam_hdr_fill_hrecs and in turn sam_hrecs_update_hashes, where duplicate check is made.

For cram, there is no added overhead as fill_hrecs is already in use.
For sam and bam, the sanitise update adds as overhead which is required to identify the duplicates.
For sam, the check occurs during normal parsing in sam_hdr_create and sanitise update is redundant.
For bam, fill_hrecs update is required to identify the duplicates. sanitise seem to be the best common place to have this.