Feature/leading slash params

[xqms]

Discovered that roslaunch ignores leading slashes in <param> tags - but only when inside a node!

<launch>
        <param name="~private" value="test" />

        <group ns="group_ns">
                <param name="relative" value="test" />
                <param name="/group_ns_absolute" value="test" />
        </group>

        <node name="joy" pkg="joy" type="joy_node">
                <param name="relative" value="test" />
                <param name="/joy_absolute" value="test" />
        </node>
</launch>

Here, roslaunch will create /group_ns_absolute but /joy/joy_absolute - go figure.

This PR creates a unit test assertion for that case and creates a special in-node mode for parsing params.

[xqms]

Added a warning message if this occurs - as I feel it can be quite confusing.

[xqms]

@ros-pull-request-builder retest this please