Added draft code for an assertion context

	modified:   ocaml/assertion_table.ml
	modified:   ocaml/feature_table.ml
	modified:   ocaml/feature_table.mli
	modified:   ocaml/term.ml
Added experimental code for 'Assertion_context' based on 'Term_table'

The assertion context is already filled but informantion is not yet extracted
from it. The interface between the prover and the assertion context has to be
reworked and defined well.

	modified:   ocaml/assertion_table.ml
	modified:   ocaml/term.ml
	modified:   ocaml/term_table.ml