Improvements to the alignments export. This is almost working now...

* [IIMB,FIX] rdf_load cannot load multiple files into the same graph (reverted).
* [IOTW_ALIGNMENTS_EXPORT,FIX] Superfluous calculation of the number of non-nil pairs.
* [IOTW_ALIGNMENTS_EXPORT,FIX] Relocated the extraction of the empty key (revert).
* [IOTW_ALIGNMENTS_EXPORT,IOTW_EXPORT,SC] Improved the calculation of the lower and higher approximation (and the accuracy), using node/5 facts.
* [IOTW_ALIGNMENTS_EXPORT,FIX] Accuracy division calculation could be divided by zero.