replaced PNG images with SVGs

Author: bergos

interface-spec.html

@@ -88,7 +88,7 @@ <h2>Design elements and principles</h2>
 <section>
   <h2>Data interfaces</h2>
 
-  <img src="img/data_diagram.png" alt="UML data interface diagram" style="width: 100%;">
+  <img src="img/data_diagram.svg" alt="UML data interface diagram" style="width: 100%;">
 
 
   <section data-dfn-for="Term">
@@ -365,7 +365,7 @@ <h2>Stream interfaces</h2>
     streams.
   </p>
 
-  <img src="img/stream_diagram.png" alt="UML data interface diagram" style="width: 100%;">
+  <img src="img/stream_diagram.svg" alt="UML data interface diagram" style="width: 100%;">
 
   <section data-dfn-for="Stream">
     <h3><dfn>Stream</dfn> interface</h3>