grid properties: introduce initializer objects

this makes it possible to define more complicated initializers for
grid properties. (needed for example for keywords like ISGU.) This
patch does not introduce them yet, but only adds the necessary
infrastructure.

notes:

 - to get around cyclic definitions in template classes, the
   EclipseState is changed to a (defaulted) template parameter which
   causes the compiler to only look up the class definition at
   instantiation time.
 - using std::shared_ptr in the property initializers would lead to
   cyclic references which would cause memory leaks. To avoid that,
   plain old references where used in most places. I think this is
   okay as the GridProperty objects should be considered internal to
   EclipseState and thus the EclipseState object should live at least
   as long as GridProperty objects...
grid properties: implement a table-lookup initializer for the endpoint scaling keywords