This function takes a flex_prms object and modifies the (linear) internal list so that a parameter is set as equal across multiple conditions, according to the instruction formula (i.e., ' prm ~! conda + condb)
flex_restrain_prms(flex_prms_obj, formula_instr)a modified flex_prms object with an updated (linear) internal list
flex_prms object
a string referring to "restrain" (see modify_flex_prms)