Add built ratio

This MR aims to create the built_up_ratio field in obm_built_area_assessments

\approve @fd

migrations/00010_add-built-ratio-field.sql

+-- Add the `obm_built_area_assessments.built_up_ratio` field
+
+
+ALTER TABLE public.obm_built_area_assessments
+ADD built_up_ratio FLOAT;
\ No newline at end of file