Relax EPS Activation Criterion

Existence of certain EPS keywords (SWL, SGCR, others) is sufficient
to activate the end-point scaling option.  The deck does not *need*
to have the ENDSCALE keyword in these cases, but it is recommended
that ENDSCALE nevertheless be used.

On the other hand, certain other keywords do require the presence of
ENDSCALE so record this in "requires" clauses.

tests/parser/FieldPropsTests.cpp

@@ -46,8 +46,9 @@
 #include <opm/parser/eclipse/EclipseState/Grid/SatfuncPropertyInitializers.hpp>
 #include <opm/parser/eclipse/EclipseState/Runspec.hpp>
 
-#include "src/opm/parser/eclipse/EclipseState/Grid/FieldProps.hpp"
+#include <opm/common/utility/OpmInputError.hpp>
 
+#include "src/opm/parser/eclipse/EclipseState/Grid/FieldProps.hpp"
 
 using namespace Opm;
 
@@ -677,6 +678,60 @@ OPERATE
     }
 }
 
+BOOST_AUTO_TEST_CASE(EPS_Props_Inconsistent) {
+    BOOST_CHECK_THROW(const auto deck = Opm::Parser{}.parseString(R"(RUNSPEC
+DIMENS
+10 10 3 /
+
+-- No GAS keyword
+
+PROPS
+SGCR    -- Requires 'GAS'
+300*0.001 /
+)"), Opm::OpmInputError);
+
+    BOOST_CHECK_THROW(const auto deck = Opm::Parser{}.parseString(R"(RUNSPEC
+DIMENS
+10 10 3 /
+
+OIL
+GAS
+-- No ENDSCALE keyword
+
+PROPS
+SOGCR    -- Requires 'ENDSCALE'
+300*0.05 /
+)"), Opm::OpmInputError);
+
+    BOOST_CHECK_THROW(const auto deck = Opm::Parser{}.parseString(R"(RUNSPEC
+DIMENS
+10 10 3 /
+
+OIL
+-- GAS
+ENDSCALE
+/
+
+PROPS
+SOGCR    -- Requires 'GAS'
+300*0.05 /
+)"), Opm::OpmInputError);
+
+    BOOST_CHECK_THROW(const auto deck = Opm::Parser{}.parseString(R"(RUNSPEC
+DIMENS
+10 10 3 /
+
+-- OIL
+GAS
+ENDSCALE
+/
+
+PROPS
+SOGCR    -- Requires 'OIL'
+300*0.05 /
+)"), Opm::OpmInputError);
+}
+
 namespace {
     std::string satfunc_model_setup()
     {