[Eva] In External domains, adds special accessors for the main cvalue domain.

src/plugins/value/domains/cvalue/cvalue_domain.ml

src/plugins/value/domains/cvalue/cvalue_domain.mli

src/plugins/value/engine/abstractions.ml

src/plugins/value/engine/analysis.ml

src/plugins/value/engine/compute_functions.ml

src/plugins/value/engine/initialization.ml

src/plugins/value/engine/iterator.ml

src/plugins/value/engine/transfer_specification.ml

src/plugins/value/engine/transfer_stmt.ml

src/plugins/value/gui_files/gui_eval.ml

src/plugins/value/register.ml

src/plugins/value/utils/abstract.ml

src/plugins/value/utils/abstract.mli