changed: hide the templated 'has' and 'get' interfaces in FieldPropsManager

motivation: an upcoming parallel frontend to the field props manager.
templated functions cannot be virtualized, and thus having these exposed
would give a great chance of confusing the users in the downstream
code, where properties would be caught from the (potentially) empty
backend instead of from the frontend.

tests/parser/ThresholdPressureTest.cpp

@@ -235,7 +235,7 @@ struct Setup
 BOOST_AUTO_TEST_CASE(ThresholdPressureDeckHasEqlnum) {
     Setup s(inputStrWithEqlNum);
 
-    BOOST_CHECK(s.fp.has<int>("EQLNUM"));
+    BOOST_CHECK(s.fp.has_int("EQLNUM"));
 }
 
 BOOST_AUTO_TEST_CASE(ThresholdPressureTest) {