some sql classes refactorings like column and table structs to classes

test/backends/SchemaTest.cpp

@@ -14,7 +14,7 @@ using namespace matador::test;
 
 TEST_CASE_METHOD(SchemaFixture, "Test schema one-two-many", "[schema]") {
     using namespace matador::test;
-    matador::orm::schema repo(pool/*, "NoopSchema"*/);
+    orm::schema repo(pool, "NoopSchema");
 
     auto result = repo.attach<department>("departments")
        .and_then( [&repo] { return repo.attach<employee>("employees"); } );