return the type of variables of a given AMPL problem
var_type = ampl_get_type(asl)
a structure which handles the pointer to the problem
a string which contains one charactere by variable. The meaning of each character is:
'n': nonlinear variable
'a': linear arcs
'o': other linear
'b': binary variable
'i': integer variable
Return the type of variables of a given AMPL problem.