Given a string 'xxx=[yy1 sep sep yyn]', define the vector ['yy1'; ;'yyn']
EXAMPLE
str2vec('a=b,c,d',',')// returns //!b !//!c !//!d !grocer_coef=str2vec(grocer_coef)// Example taken from nls: str2vec is used to transform into a vector the list of coefficient names given by the user.