SMT2 backend: use FPA theory with --cprover-smt2

This enables the use of the FloatingPoint theory when using cprover_smt2.
The resulting SMT-LIB files are much smaller, and easier to read.

Author: kroening

src/solvers/smt2/smt2_conv.cpp

@@ -85,6 +85,7 @@ smt2_convt::smt2_convt(
     break;
 
   case solvert::CPROVER_SMT2:
+    use_FPA_theory = true;
     use_array_of_bool = true;
     use_as_const = true;
     use_check_sat_assuming = true;