ProxyGen XSD - Split switch into its own conditional element

[brine]

      <conditional>
        <switch >
          <case />
          <default />
        </switch>
      </conditional>
      <conditional>
        <if />
        <elseif  />
        <else />
      </conditional>

Currently, the switch conditional is nested inside the conditional element. Switch should be moved up as its own element alongside conditional (which exclusively becomes a container for if/else types).

This allows us to better restrict the amount of if and else elements included within conditional, as it won't have to expect to find a switch element anymore.