First,construct a Bmi class,using separate code concept,which has 
height,weight for data member,four mamber function getX2,setX2 
for excess to data member,a calbmi() to solve for bmi value,
and a category() to decide the category.

Then using ifstream to read data from the file "bmi",
call the member function set(height),set(weight),calbmi().

And finally,calling category() to set the category of eacg data.