replace a "true' string by another one in a string
[txt,true_obj]=strsubst_trueobj(txt,obj1,obj2,before,after,suppr_blank,opt_convstr)
vi = a string
obj1 = a string to substitue in txt
obj2 = the new string
before = a string vector: what character must be found before vi
after = a string vector: what character must be found after vi
suppr_blank = a booelan indicating whether blanks count or not (optional: default=%f)
convstr = a booelan indicating whether the searched object must be converted to lower case or not (optional: default=%f)
txt = the new txt
true_obj = a boolean indicating whether the object has been found