Allow complex terms

[RichardMoot]

The beta version allows a predicate to have only variable or constant terms. Some of the more subtle applications of first-order linear logic require complex terms. For example, the treatment of associativity and scope restrictions in my 2014 Lambek paper uses "integer-like" terms (of the forms s(X), s(s(X), etc.)

[RichardMoot]

Done! Still need to add a few more illustrations of the applications to ll1_grammar.pl