sylva-units: refactor how uninterpreted unit_templates is preserved for unit-def-from-templates

This MR is actually an implementation idea of @feleouet (commit author set to François Eleouet <francois.eleouet@orange.com>).

This is an iso-functional code simplification of the code that is meant to preserve .unit_templates from being interpreted too early, so as to allow it's interpretation only in the context of a given unit, to allow the use of the special .Values._unit_name_ variable.

(!2054 (merged) depends on this MR)