Proper getting of read/write functions in loopconfig object.

The read and write function for an STL collection proxy are not necessarily
the same (in particular for an associative container).  So when initializing
the loop config, we need to make sure to use the grab the right ones.
Fix the use of iterator for associative container

The iterator for read and writing associative container are different
(for reading we first need to 'stage' the data), so we need to make
sure to properly initialize them.