Preprocessor: Do not simplify constant equations which have a tag attached

In particular, in the case of an 'mcp' tag, this could lead to a convergence
failure.

Also modify the lmmpc/rbcii.mod test so that it fails if convergence is not
obtained, in order to detect such regressions earlier.

Closes: #1697

This preprocessor update also includes the following changes:
— removal of license.txt, now merged in back in the main Dynare one
– various code simplifications and modernizations in the macro processor
— minor code simplification in parsing of the --+ options: … +-- line

preprocessor

@@ -1 +1 @@
-Subproject commit a8b7bd31c58f3afe4f35b90fe56f0ae8509efaea
+Subproject commit bef537d40accd597c19f112471974b726c9c9890

tests/lmmcp/rbcii.mod

@@ -53,7 +53,11 @@ end;
 steady;
 
 perfect_foresight_setup(periods=400);
-perfect_foresight_solver(lmmcp, maxit=200);
+perfect_foresight_solver(lmmcp, maxit=200, no_homotopy);
+
+if ~oo_.deterministic_simulation.status
+    error('Convergence not obtained')
+end
 
 n = 40;