Place the clear button in the same row as the radio buttons

Creates a single row of "tools" differentiated from the next row of "data
selection" dropdowns. Addresses one part of #64.

docs/_javascript/main.js

@@ -123,11 +123,7 @@ window.addEventListener('DOMContentLoaded', (event) => {
           d3.select("#markdown-url").property('value'))
         );
 
-      var clearButton = d3.select("#line_plot")
-        .insert("button", "svg")
-        .text("clear selections")
-        .attr("id", "clearButton")
-        .classed("button", true)
+      var clearButton = d3.select("#clearButton")
         .on('click', clearbuttonchange);
 
       var conditiondropdown = d3.select("#line_plot")

docs/index.html

@@ -35,6 +35,7 @@ <h4>Instructions:</h4>
         <p>
           <input type='radio' id="select" name="mode" value="select" checked /> select
           <input type='radio' id="deselect" name="mode" value="deselect" /> deselect
+          <button id="clearButton" class="button">clear selections</button>
         </p>
 
         <div id="line_plot" class="row"></div>