printing of symbolic expressions needs improvement

[Stephen Siegel]

The symbolic expressions, when printed, do not contain parentheses, so one cannot tell how the operations are grouped. For example:

0<=X0&&X0*-1+2==0

0<=X0*-1+1

Look at the Expression classes in the model package to see one way to handle this easily. Each expression class provides two methods to return a string, one which is used whenever the expression has to occur as a subgroup inside of another expression. You can just copy what is done there.

[ywei]

Fixed. Parenthesis added.