big reorganisation of asserts, so that:

0) asserts are only done in the public API, except for a few ones explicitly
   named eigen_internal_assert.
1) internal asserts are disabled unless EIGEN_INTERNAL_DEBUGGING is defined.
   This limits the impact of debugging on performance.
2) no 'unused argument' warnings anymore when compiling with -DNDEBUG

test/main.h

@@ -27,6 +27,8 @@
 #define EIGEN_TEST_MAIN_H
 
 #include <QtTest/QtTest>
+
+#define EIGEN_INTERNAL_DEBUGGING
 #include "../src/Core.h"
 
 #include <cstdlib>