Electricity signals (part 4) (#144)

Part of #36 

Changes:
- Add missing DE BOStrab signals
- Render DE HHA signals
- Add CH FDV signals and icons 
- Add LU CFL signals and icons
- Add BE signals and icons
- Add PL tram signals and icons
- Add ES signals and icons

From https://de.wikipedia.org/wiki/Fahrleitungssignale and
https://wiki.openstreetmap.org/wiki/DE:OpenRailwayMap/Tagging_in_Switzerland#Signale_f%C3%BCr_den_elektrischen_Betrieb,
https://fr.wikipedia.org/wiki/Signalisation_ferroviaire_luxembourgeoise

proxy/js/styles.mjs

@@ -719,8 +719,8 @@ const electrificationColor = (voltageProperty, frequencyProperty) => ['case',
   ['all', ['==', ['get', frequencyProperty], 50], ['==', ['get', voltageProperty], 25000]], color_25kv_50hz,
   ['all', ['==', ['get', frequencyProperty], 60], ['==', ['get', voltageProperty], 20000]], color_20kv_60hz,
   ['all', ['==', ['get', frequencyProperty], 50], ['==', ['get', voltageProperty], 20000]], color_20kv_50hz,
-  ['all', ['<', 16.665, ['get', frequencyProperty]], ['<', ['get', frequencyProperty], 16.675], ['==', ['get', voltageProperty], 15000]], color_15kv_16_67hz,
-  ['all', ['<', 16.65, ['get', frequencyProperty]], ['<', ['get', frequencyProperty], 16.75], ['==', ['get', voltageProperty], 15000]], color_15kv_16_7hz,
+  ['all', ['!=', ['get', frequencyProperty], null], ['<', 16.665, ['get', frequencyProperty]], ['<', ['get', frequencyProperty], 16.675], ['==', ['get', voltageProperty], 15000]], color_15kv_16_67hz,
+  ['all', ['!=', ['get', frequencyProperty], null], ['<', 16.65, ['get', frequencyProperty]], ['<', ['get', frequencyProperty], 16.75], ['==', ['get', voltageProperty], 15000]], color_15kv_16_7hz,
   ['all', ['==', ['get', frequencyProperty], 60], ['==', ['get', voltageProperty], 12500]], color_12_5kv_60hz,
   ['all', ['==', ['get', frequencyProperty], 25], ['==', ['get', voltageProperty], 12000]], color_12kv_25hz,
   ['all', ['==', ['get', frequencyProperty], 0], ['!=', ['get', voltageProperty], null], ['>', ['get', voltageProperty], 3000]], color_gt3kv_dc,
@@ -2700,7 +2700,7 @@ const layers = {
     {
       id: 'electrification_signals_direction',
       type: 'symbol',
-      minzoom: 16,
+      minzoom: 15,
       source: 'openrailwaymap_electrification',
       'source-layer': 'electrification_signals',
       filter: ['all',
@@ -2734,7 +2734,7 @@ const layers = {
       ['get', 'feature'],
       {
         type: 'symbol',
-        minzoom: 16,
+        minzoom: 15,
         source: 'openrailwaymap_electrification',
         'source-layer': 'electrification_signals',
         layout: {