Thin-film improvements (#167)

This addresses the following:

- Implementing thin-film interference in combination with F82-tint metallic Fresnel #79
- For consistency, every section should have a "weight" parameter. #134
- Thin film in micrometers divided by 2 for normalized parameter range #150

examples/open_pbr_default.mtlx

@@ -36,6 +36,7 @@
     <input name="coat_anisotropy" type="float" value="0.0" />
     <input name="coat_rotation" type="float" value="0.0" />
     <input name="coat_ior" type="float" value="1.6" />
+    <input name="thin_film_weight" type="float" value="0.0" />
     <input name="thin_film_thickness" type="float" value="0" />
     <input name="thin_film_ior" type="float" value="1.5" />
     <input name="emission_luminance" type="float" value="0.0" />

examples/open_pbr_pearl.mtlx

@@ -13,7 +13,8 @@
     <input name="coat_weight" type="float" value="1" />
     <input name="coat_roughness" type="float" value="0.15" />
     <input name="coat_ior" type="float" value="1.68" />
-    <input name="thin_film_thickness" type="float" value="420" />
+    <input name="thin_film_weight" type="float" value="1.0" />
+    <input name="thin_film_thickness" type="float" value="0.420" />
     <input name="thin_film_ior" type="float" value="2" />
   </open_pbr_surface>
 </materialx>

examples/open_pbr_soapbubble.mtlx

@@ -7,7 +7,8 @@
     <input name="specular_roughness" type="float" value="0" />
     <input name="specular_ior" type="float" value="1.0" />
     <input name="transmission_weight" type="float" value="1.0" />
-    <input name="thin_film_thickness" type="float" value="500" />
+    <input name="thin_film_weight" type="float" value="1.0" />
+    <input name="thin_film_thickness" type="float" value="0.5" />
     <input name="thin_film_ior" type="float" value="1.4" />
     <input name="geometry_thin_walled" type="boolean" value="true" />
   </open_pbr_surface>