Revert "Data copying takes an iterator"

The original rationale for commit c39d367 was to pass a pointer since
this was all we got from one of the APIs. However, just after the code
was changed to copy the data anyway since it would have to be converted
from double to float.

Reverting actually serves a purpose; as @bska pointed out, taking
&data[0] of an empty vector is undefined, whereas it now checks the
size and assert here (and not in the standard library) on that condition.

opm/core/io/eclipse/EclipseWriter.cpp

@@ -115,7 +115,7 @@ struct EclipseKeyword : public EclipseHandle <ecl_kw_type> {
         : EclipseHandle <ecl_kw_type> (
               ecl_kw_alloc (name.c_str(), data.size (), type ()),
               ecl_kw_free) {
-        copyData (&data[0], data.size (), offset, stride);
+        copyData (data, offset, stride);
     }
 
     /// Special initialization from double-precision array which
@@ -141,7 +141,7 @@ struct EclipseKeyword : public EclipseHandle <ecl_kw_type> {
                             type ()),
               ecl_kw_free) {
         const std::vector <T>& data = parser.getValue <T> (name);
-        copyData (&data[0], data.size (), 0, 1);
+        copyData (data, 0, 1);
     }
 
     /// Constructor for optional fields
@@ -167,10 +167,12 @@ private:
 
     /// Helper function that is the meat of the constructor
     template <typename U>
-    void copyData (const U* data,
-                   const int num,
+    void copyData (std::vector <U> data,
                    const int offset,
                    const int stride) {
+        // number of elements we can possibly take from the vector
+        const int num = data.size ();
+
         // range cannot start outside of data set
         assert(offset >= 0 && offset < num);
 
@@ -205,7 +207,7 @@ EclipseKeyword <float>::EclipseKeyword (
                         type ()),
           ecl_kw_free) {
     const std::vector <double>& data = parser.getValue <double> (name);
-    copyData (&data[0], data.size (), 0, 1);
+    copyData (data, 0, 1);
 }
 
 /// Provide only the float version, since that is the one for which
@@ -220,7 +222,7 @@ EclipseKeyword <float>::EclipseKeyword (
     : EclipseHandle <ecl_kw_type> (
           ecl_kw_alloc (name.c_str(), data.size (), type ()),
           ecl_kw_free) {
-    copyData (&data[0], data.size (), offset, stride);
+    copyData (data, offset, stride);
 }
 
 /**