Require ENDSCALE When JFUNC Entered

We effectively ignore JFUNC data unless end-point scaling is
activated through the ENDSCALE keyword in the RUNSPEC section.
Alert the user to this fact by terminating the simulation run if the
case uses JFUNC without ENDSCALE present in RUNSPEC.  In this case,
issue a diagnostic message of the form

    Error: Unrecoverable errors while loading input: Problem with keyword JFUNC
    In CASE.DATA line 125
    Incompatible keyword combination: JFUNC declared, but ENDSCALE is missing.

python/tests/data/JFUNC.DATA

@@ -5,6 +5,10 @@ TITLE
 
 DIMENS
  10 10 10 /
+
+ENDSCALE
+/
+
 GRID
 DX
 1000*0.25 /

python/tests/test_state.py

@@ -156,6 +156,8 @@ SATNUM
         jfunc_gas = """
 DIMENS
  10 10 10 /
+ENDSCALE
+/
 GRID
 DX
 1000*0.25 /

src/opm/input/eclipse/share/keywords/000_Eclipse100/J/JFUNC

@@ -4,6 +4,7 @@
     "GRID"
   ],
   "size": 1,
+  "requires": ["ENDSCALE"],
   "items": [
     {
       "name": "FLAG",

src/opm/input/eclipse/share/keywords/000_Eclipse100/J/JFUNCR

@@ -7,6 +7,7 @@
     "keyword": "TABDIMS",
     "item": "NTSFUN"
   },
+  "requires": ["ENDSCALE"],
   "items": [
     {
       "name": "J_FUNCTION",