Renamed write_slab to write_rslab. And added clarifying comment for this

method.

cpp/field.hpp

@@ -89,7 +89,7 @@ class field
                 const std::string field_name,
                 const int iteration,
                 const bool read = true);
-        int write_slab(
+        int write_rslab(
                 const std::string fname,
                 const std::string field_name,
                 const int iteration,

cpp/field_io.cpp

@@ -345,10 +345,14 @@ int field<rnumber, be, fc>::io(
     return EXIT_SUCCESS;
 }
 
+ /** Writes a slab of a real field to a file. The slab starts at index 0 
+  * and ends at thickness in the lowest dimension (z-direction). 
+  * All other dimensions are kept untouched.
+ */
 template <typename rnumber,
           field_backend be,
           field_components fc>
-int field<rnumber, be, fc>::write_slab(
+int field<rnumber, be, fc>::write_rslab(
         const std::string fname,
         const std::string field_name,
         const int iteration,