get speed in pint library for plot_deflected_shape function

[ViniciusTxc3]

Now, it's possible receive the speed in "RPM" for function plot_deflected_shape(). As exemple:

import ross as rs
from ross.units import  Q_
import numpy as np

rotor = rs.rotor_example()
speed = np.linspace(0, 1000, 101)
response = rotor.run_unbalance_response(
                                        node=3,
                                        unbalance_magnitude=10.0,
                                        unbalance_phase=0.0,
                                        frequency=speed)
response.plot_deflected_shape(speed=Q_(4774.64829, 'RPM')).show()

This PR, resolve issue #1137 and discussion #1136 .