add utility classes for some common keywords

the idea is to create a lightweight wrapper objects around
DeckKeywords which allow more convenient and more readable access to
the data. E.g. instead of

  std::vector outerColumnNames{"RV", "P", "BG", "MUG"};
  std::vector innerColumnNames{"P", "BG", "MUG"};
  Opm::FullTable pvtgTable(deck->getKeyword("PVTG"),
                           outerColumnNames, innerColumnNames);
  pvtgTable->getOuterTable()->getColumn(1);

one now better uses

  Opm::PvtgTable pvtgTable(deck->getKeyword("PVTG"));
  pvtgTable->getOuterTable()->getPressureColumn();

the idea for the other keywords is similar.

testdata/integration_tests/SWOF/SWOF.txt

@@ -1,19 +0,0 @@
-TABDIMS
--- NTSFUN NTPVT NSSFUN NPPVT NTFIP NRPVT
-        1     1     30     1     1     1 /
-
-
-SWOF
---  SWAT   KRW   KRO   PCOW
-   0.1       0   1.00     0
-   0.15   0.06   1.00     1
-   0.2    0.11   1.00     2
-   0.3    0.22   0.87     3
-   0.4    0.33   0.73     4
-   0.5    0.44   0.60     5
-   0.6    0.56   0.47     6
-   0.7    0.67   0.33     7
-   0.8    0.78   0.20     8
-   0.9    0.89   0.07     9
-   1.0    1.00   0.00     10
-/