tdf#50916 : Refactor table1.cxx wherever there is column access

This is still a WIP for table1.cxx, but want to get some comments
as I proceed.

Added back() method to ScColContainer to avoid aCol[aCol.size()-1]
usages.

Looks like we will need to initialize column container with at least
one column in the ScTable constructor because we need to use a lot
of "aCol.size() - 1" expression.

Change-Id: I5f0fa3d2daed67cb623641b273d2727561ab3d49
Reviewed-on: https://gerrit.libreoffice.org/31125Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

sc/inc/colcontainer.hxx

@@ -56,6 +56,18 @@ public:
     }
 
     void Clear();
+
+    const ScColumn& back() const
+    {
+        assert(aCols.size() > 0);
+        return *aCols.back();
+    }
+
+    ScColumn& back()
+    {
+        assert(aCols.size() > 0);
+        return *aCols.back();
+    }
 };