user specified precision in rational printing

[lgwagner]

provide a user flag to specify the precision of rationals.

I'm thinking: -precision INT

the INT argument will produce the underlying fraction if the argument is 0 or N places of precision in a decimal format (up to some reasonable level, like 9). I think we should default N to 3 so it mimics current behavior by default.

Notes:

• we used to provide a printout of the fractions that represented rationals but it became very hard to relate two values in a CEX (i.e. is 4388234/2342 > 2378823/273 ) which was easy to do for a couple of values but tedious for numerous values.
• loss of precision in the current method has its own drawbacks