Validate entered number to be integer or "half"
<p>I am getting input from the user and it needs to be either a whole number or some multiple of <code>0.5</code>.</p>

<p>For example, <code>7</code>, <code>23</code> and <code>5.5</code> are all valid but <code>3.1</code> and <code>2.7</code> are not.</p>

<p>In other words, if the user keys in <code>2.3</code>, there will be an alert telling them that they can only key in <code>2.5</code>.</p>

<p>How do I go about performing this validation?</p>
