Example:

auto const xml = R"urdf(
    <robot name="test">
        <link name="base">
        </link>
        <link name="link1">
            <inertial>
            <origin xyz="0. 0. 0."/>
            <mass value="1."/>
            <inertia ixx="1." ixy="0." ixz="0." iyy="1." iyz="0." izz="1."/>
            </inertial>
        </link>
        <joint name="joint0" type="continuo

TODOs:

• Create a simple example for the C++ API online.

#546 adds a lengthy pattern that gets the inverse of a state before composing it. Instead we should have an inverse composition (i.e. subtraction) function implemented between states for brevity.

@gilwoolee

Members and scientific notation has not been referenced or documented. Sometimes it is quite hard to guess what variables are. We should document them in each header.