Validate subgroup parsing order issues in SRDF

[sea-bass]

This causes an informative error message to be thrown in the event that SRDFs use a subgroup before it is defined. For example:

    <group name="arm_right">
        <joint name="arm_right_1_joint" />
        <joint name="arm_right_2_joint" />
        <joint name="arm_right_3_joint" />
        <joint name="arm_right_4_joint" />
        <joint name="arm_right_5_joint" />
        <joint name="arm_right_6_joint" />
        <joint name="arm_right_7_joint" />
        <joint name="arm_right_tool_joint" />
    </group>
    <group name="arm_right_torso">
        <group name="arm_right" />
        <group name="torso" /> <!-- THIS IS THE ISSUE -->
    </group>
    <group name="torso">
        <joint name="torso_lift_joint" />
    </group>

Closes #156