SDF: support plugin with repeated elements

[srmainwaring]

This PR adds support for SDF plugins that contain repeated elements (#139)

Repeated elements are specified by assigning a list to a tag in a dictionary of plugin arguments. For example running the code:

from pcg_gazebo.parsers.sdf import create_sdf_element
from pcg_gazebo.parsers.sdf import Plugin

plugin_args = dict(
    control=list([  
        dict(
            attributes=dict(
                channel="0"
            ),
            value=dict(
                jointName='steer_joint',
                useForce=1
            )
        ),
        dict(
            attributes=dict(
                channel="2"
            ),
            value=dict(
                jointName='motor_joint',
                useForce=1
            )
        )        
    ])
)

plugin = create_sdf_element('plugin')
plugin.name = 'plugin'
plugin.filename = 'libPlugin.so'
plugin.value = plugin_args
print(plugin)

ouputs

<plugin name="plugin" filename="libPlugin.so">
  <control channel="0">
    <jointName>steer_joint</jointName>
    <useForce>1</useForce>
  </control>
  <control channel="2">
    <jointName>motor_joint</jointName>
    <useForce>1</useForce>
  </control>
</plugin>

Lists of numbers and strings continue to work as before:

plugin_args = dict(
    xyz=[1.0, 2.0, 3.0],
    rpy=[0.1, 0.2, 0.3],
)

plugin = create_sdf_element('plugin')
plugin.name = 'plugin'
plugin.filename = 'libPlugin.so'
plugin.from_dict(plugin_args)
print(plugin)

<plugin name="plugin" filename="libPlugin.so">
  <xyz>1.0 2.0 3.0</xyz>
  <rpy>0.1 0.2 0.3</rpy>
</plugin>