Influence of Nitrogen on Structure, Guest Distribution and Hydrate Formation Conditions of the Mixed CO&lt;sub&gt;2&lt;/sub&gt;&amp;ndash;CH&lt;sub&gt;4&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt;&amp;ndash;CH&lt;sub&gt;4&lt;/sub&gt;&amp;ndash;N&lt;sub&gt;2&lt;/sub&gt; Gas Hydrates