SuReal: `lg-get-dict-entry` scheme primitive

* added a `lg-get-dict-entry` scheme primitive for constructing an entry for a word
    - usage: `(lg-get-dict-entry (WordNode "yes"))`
    - for upcoming SuReal usage
    - the atom types and atomic formula are extracted from the viterbi code
    - the `ReferenceNode` thing added by me awhile back is not really needed due to later changes, so removed in the same commit while I am editing that file
    - require latest link-grammar code from github

* slightly updated viterbi code
    - made it uses the LGDictReader
    - made it build-able, and the corresponding tests passing
    - kept it commented out in the CMake file

The LGDictModule perhaps should be on another directory... maybe move it when the language learning part is implemented.

It would be better to move this code from here to a new subdirectory `opencog/nlp/lg-dict`.