Table Output: Don't Store ECL Keywords Internally

This commit switches the internal implementation of the 'tab' and
'tabdims' vectors from being a mix of ECL keyword structures and
std::vector<>s to being purely std::vector<>s.  We remove the
fwrite() member function and rather reimplement the feature in terms
of new query member functions

    const std::vector<int>& tabdims() const
    const std::vector<double>& tab() const

that return read-only references to internal data members.

tests/test_Tables.cpp

@@ -78,7 +78,7 @@ BOOST_AUTO_TEST_CASE(Test_PVTX) {
     tables.addDensity( cfg.es.getTableManager().getDensityTable( ) );
     {
         ERT::FortIO f("TEST.INIT" , std::fstream::out);
-        tables.fwrite( f );
+        fwrite(tables, f);
     }