Method openForRead opens the resource represented by this Tap instance for reading.
{@code input} value may be null, if so, sub-classes must inquire with the underlying {@link Scheme}via {@link Scheme#sourceConfInit(cascading.flow.FlowProcess,Tap,Object)} to get the properinput type and instantiate it before calling {@code super.openForRead()}.
Note the returned iterator will return the same instance of {@link cascading.tuple.TupleEntry} on every call,thus a copy must be made of either the TupleEntry or the underlying {@code Tuple} instance if they are to bestored in a Collection.
@param flowProcess of type FlowProcess
@param input of type Input
@return TupleEntryIterator
@throws java.io.IOException when the resource cannot be opened