Improvement to dynamic lexicon functionality

I have just checked in some improvements to dynamic lexicons, which should considerably reduce the number of external files created at runtime. Hopefully this will improve recognition response times, but so far I don't have a non-trivial dynamic lexicon application to test on - so I would appreciate feedback from the Ford project. In particular, please let me know at once if anything appears to be broken. If necessary, you can reverse the change by reverting the file $REGULUS/Prolog/dynamic_lexicon.pl to the previous version.