simplify use of secondary importance

The values in the raster are already normalized between 0 and 2**16,
so a simple conversion to [0, 1] will do.

Check for existance of secondary_importance table statically when
creating the SQL function. For that to work importance tables need
to be created before the functions.

lib-sql/tables.sql

@@ -276,7 +276,7 @@ CREATE SEQUENCE file start 1;
 
 -- null table so it won't error
 -- deliberately no drop - importing the table is expensive and static, if it is already there better to avoid removing it
-CREATE TABLE wikipedia_article (
+CREATE TABLE IF NOT EXISTS wikipedia_article (
     language text NOT NULL,
     title text NOT NULL,
     langcount integer,
@@ -290,15 +290,12 @@ CREATE TABLE wikipedia_article (
     wd_page_title text,
     instance_of text
 );
-ALTER TABLE ONLY wikipedia_article ADD CONSTRAINT wikipedia_article_pkey PRIMARY KEY (language, title);
-CREATE INDEX idx_wikipedia_article_osm_id ON wikipedia_article USING btree (osm_type, osm_id);
 
-CREATE TABLE wikipedia_redirect (
+CREATE TABLE IF NOT EXISTS wikipedia_redirect (
     language text,
     from_title text,
     to_title text
 );
-ALTER TABLE ONLY wikipedia_redirect ADD CONSTRAINT wikipedia_redirect_pkey PRIMARY KEY (language, from_title);
 
 -- osm2pgsql does not create indexes on the middle tables for Nominatim
 -- Add one for lookup of associated street relations.