[feature] Add optional backwards mapping, for consistency with older version.

assigned to @jjd1c23

ACMC would need backwards mapping to be consistent with previous concepts 3.2.10. Suggestion 2 releases:

• Concepts 4.0.0 - without backwards mapping
• Concepts 4.1.0 - with backwards mapping

allows for choice in the future.

Backwards mapping flag now implemented ... however running diff against all concepts still shows more read2 and snomed codes are being added then before.

created branch 61-feature-add-optional-backwards-mapping-for-consistency-with-older-version to address this issue