JSON keyword definitions: add a "sections" item

this which specifies the allowed sections of the keyword. (surprise!)

testdata/parser/EQLDIMS2

@@ -1,2 +1,7 @@
-{"name" : "EQLDIMS"  , "size" : 1 , "items": 
- [{"name" : "NEW"          , "value_type" : "INT" , "default" : 1}]}
+{
+  "name" : "EQLDIMS",
+  "size" : 1,
+  "sections": ["RUNSPEC"],
+  "items": 
+  [{"name" : "NEW"          , "value_type" : "INT" , "default" : 1}]
+}