Cleaned yices wrapping code.
Cleaned yices wrapping code.

Now the values are checked for representability and an exception is
risen if the value is not represenatble.

This is a more principled fix for #77

chubbymaggie/pysmt,agriggio/pysmt,pysmt/pysmt,chubbymaggie/pysmt,idkwim/pysmt,pysmt/pysmt,idkwim/pysmt,agriggio/pysmt