"example_mpm_twoway_coupling" of newton has error

[808-code]

I only added the code to extract and display the real-time interaction force curve in this example, and did not modify the main part of the code But the running results showed that the interaction force between a 75kg object and sand after colliding and stabilizing remained at around 3800N.
test.py

[momo-van]

@808-code can you please double check and see if this is still the case.

@gdaviet for viz.