Explicit end marker for models printed in the CVC language (#3934)

Fixes https://github.com/CVC4/cvc4-wishues/issues/9.

When communicating with CVC4 using pipes and the CVC language, it was not possible to determine when all the lines of a model have been printed.

This change adds begin and end markers as the example below:
```
MODEL BEGIN
x : INT = -3; 
y : INT = 0;  
z : INT = 0;
MODEL END;
```

1 files changed, 17 insertions, 0 deletions

diff --git a/src/printer/cvc/cvc_printer.cpp b/src/printer/cvc/cvc_printer.cpp
index 86dd31da2..79a8904cd 100644
--- a/src/printer/cvc/cvc_printer.cpp
+++ b/src/printer/cvc/cvc_printer.cpp
@@ -1174,6 +1174,23 @@ void DeclareFunctionCommandToStream(std::ostream& out,
 
 }  // namespace
 
+void CvcPrinter::toStream(std::ostream& out, const Model& m) const
+{
+  // print the model comments
+  std::stringstream c;
+  m.getComments(c);
+  std::string ln;
+  while (std::getline(c, ln))
+  {
+    out << "; " << ln << std::endl;
+  }
+
+  // print the model
+  out << "MODEL BEGIN" << std::endl;
+  this->Printer::toStream(out, m);
+  out << "MODEL END;" << std::endl;
+}
+
 void CvcPrinter::toStream(std::ostream& out,
                           const Model& model,
                           const Command* command) const