relax strictness of the section handling and fix/work around a few bugs

We now do not require the sections to be correctly ordered and the
presence of the mandatory sections since even the unit tests did not
always specify all mandatory sections, which lead to a section
containing the rest of the deck if one of the expected next sections
was not specified.

also it seems like the DeckKeyword::getDeckIndex() does not correctly
work in some situations which lead

assert(deck->getKeyword(i)->name() == startKeyword);

to fail in the Section::populateKeywords() method. now the deck is
always sequentially traversed to find the position of a section's
start keyword. (This is necessary anyway if one wants to make sure
that the deck does not specify the same section more than once, a
feature which this patch also adds.)
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
Grid properties: process the integer grid properties fully before starting with the double ones

this is needed because the initializers of double grid properties
sometimes need the final value of integer properties (like SATNUM,
etc)
EP-scaling initializer: use the integer grid properties instead of the keyword directly

this is required as sometimes these fields are processed and sometimes
the keyword corresponding to the field is not present in the deck:

```
SATNUM
1*1 /

COPY
'SATNUM' 'IMBNUM' /
```

(this deck will have an IMBNUM property but no IMBNUM keyword.)