changed name for scalar function function mediation
Renamed simulation for sfs

Originally, the simulation that I used for scalar-function-scalar is
named 'Simulation_model_based_fRegress.Rmd', I changed the name to the
new convention - 'Simulation_scalar_function_scalar.Rmd'
Modified sff simulation

Modified the scalar-function-function simulation
Simulation to troubleshoot mediation model

this simulation is used to see if I can estimate beta accurately from
the mediation set up that I have. It uses fgam instead of fRegress