formula_io.pl, logic.pl: check propositionnal atom when loading.

prop_atom/1 becomes an exported predicate in logic.pl. It is used in
formula_io when loading raw specs.

replace almost all atom(A) by prop_atom(A) in generator.pl

adjust prop_atom(A) definition in logic.pl: exclude
reserved terms (disj_, conj_, neg_, f_, sf_).