add a force_squared_coupling_decay option on the run_madspin function at MadGraphUtils.py

Regarding an MC request on the jira (https://its.cern.ch/jira/browse/ATLMCPROD-7467), the run_madspin function at MadGraphUtils.py is needed to modify to decay processes containing squared order of coupling. If the force_squared_coupling_decay option is False, anything is unchanged, behave as exactly the same as the current master. If the option is True, coupling syntax with ^2== or ==1 is deleted from both the Cards/proc_card_mg5.dat and the LHE file.