Initialize a flexcomp object in PyMJCF

Mujoco 3.0 introduced [flexcomp](https://mujoco.readthedocs.io/en/stable/XMLreference.html#body-flexcomp) object for modelling deformable obejcts.

However trying to create a Prop using

```python
class Cloth(prop.Prop):
"""Simple cloth prop that consists of a MuJoco flexcomp."""

def _build(self, *args, **kwargs):
del args, kwargs
mjcf_root = mjcf.RootElement()

mjcf_root.extension.add('plugin', plugin="mujoco.elasticity.shell")

# Props need to contain a body called prop_root
mjcf_root.worldbody.add('body', name='prop_root')

cloth_object = mjcf.from_file('cloth.xml')
mjcf_root.attach(cloth_object)

super()._build('cloth', mjcf_root)
```
Errors out as flexcomp is not recoginzed as part of schema.

I modified schema.xml(which is part of mjcf under dm_control) and after adding all the tags and attibutes that I am using in an xml such as below

```xml

```
I can get the xml to be parsed and get a Prop object. I looked at composite as an example and copied over the spec as described in mujoco XML reference.

However I cannot get the simulation to run and it fails at this line `self._qp_mapper = _CartesianVelocityMapper(qp_params)` in `dm_robotics/moma/effectors/cartesian_6d_velocity_effector.py`. The error is `Process finished with exit code 139 (interrupted by signal 11:SIGSEGV)` which just sounds like it crashed

I cannot understand why. The whole file which I am running is

```python
"""Minimal working example of the dm_robotics Panda model."""
import dm_env
import numpy as np
from dm_env import specs

from dm_robotics.panda import environment
from dm_robotics.panda import parameters as params
from dm_robotics.panda import run_loop, utils

from dm_robotics.panda import arm_constants

import math
from dm_control import mjcf
from dm_robotics.moma import entity_initializer, prop
from dm_control.composer.variation import distributions, rotations

class Ball(prop.Prop):
"""Simple ball prop that consists of a MuJoco sphere geom."""

def _build(self, *args, **kwargs):
del args, kwargs
mjcf_root = mjcf.RootElement()
# Props need to contain a body called prop_root
body = mjcf_root.worldbody.add('body', name='prop_root')
body.add('geom',
type='sphere',
size=[0.04],
solref=[0.01, 0.5],
mass=1,
rgba=(1, 0, 0, 1))
super()._build('ball', mjcf_root)

class Cloth(prop.Prop):
"""Simple cloth prop that consists of a MuJoco flexcomp."""

def _build(self, *args, **kwargs):
del args, kwargs
mjcf_root = mjcf.RootElement()

mjcf_root.extension.add('plugin', plugin="mujoco.elasticity.shell")

# Props need to contain a body called prop_root
mjcf_root.worldbody.add('body', name='prop_root')

cloth_object = mjcf.from_file('cloth.xml')
mjcf_root.attach(cloth_object)

super()._build('cloth', mjcf_root)

class Agent:
"""The agent produces a trajectory tracing the path of an eight
in the x/y control frame of the robot using end-effector velocities.
"""

def __init__(self, spec: specs.BoundedArray) -> None:
self._spec = spec

def step(self, timestep: dm_env.TimeStep) -> np.ndarray:
"""Computes velocities in the x/y plane parameterized in time."""
time = timestep.observation['time'][0]
r = 0.1
vel_x = r * math.cos(time) # Derivative of x = sin(t)
vel_y = r * ((math.cos(time) * math.cos(time)) -
(math.sin(time) * math.sin(time)))
action = np.zeros(shape=self._spec.shape, dtype=self._spec.dtype)
# The action space of the Cartesian 6D effector corresponds
# to the linear and angular velocities in x, y and z directions
# respectively
action[0] = vel_x
action[1] = vel_y
return action

if __name__ == '__main__':
# We initialize the default configuration for logging
# and argument parsing. These steps are optional.
utils.init_logging()
parser = utils.default_arg_parser()
args = parser.parse_args()

# Use RobotParams to customize Panda robots added to the environment.
robot_params = params.RobotParams(robot_ip=args.robot_ip, actuation=arm_constants.Actuation.CARTESIAN_VELOCITY)
panda_env = environment.PandaEnvironment(robot_params)

ball = Ball()
cloth = Cloth()
props = [ball, cloth]

panda_env.add_props(props)
initialize_props = entity_initializer.prop_initializer.PropPlacer(
props,
position=distributions.Uniform(-.5, .5),
quaternion=rotations.UniformQuaternion())

panda_env.add_entity_initializers([initialize_props])

with panda_env.build_task_environment() as env:
# Print the full action, observation and reward specification
utils.full_spec(env)
# Initialize the agent
agent = Agent(env.action_spec())
# Run the environment and agent either in headless mode or inside the GUI.
if args.gui:
app = utils.ApplicationWithPlot()
app.launch(env, policy=agent.step)
else:
run_loop.run(env, agent, [], max_steps=1000, real_time=True)
```

Which is just a modification of the basic example in [dm_robotics_panda] (https://github.com/JeanElsner/dm_robotics_panda)

Commenting out creating of Cloth and adding to props list works fine.