[wp] prover name fallback

src/plugins/wp/Changelog

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/VCS.ml

src/plugins/wp/Why3Provers.ml

src/plugins/wp/Why3Provers.mli

src/plugins/wp/doc/manual/wp_plugin.tex

src/plugins/wp/register.ml

...p/tests/wp_acsl/oracle_qualif/classify_float.1.res.oracle

...p/tests/wp_acsl/oracle_qualif/classify_float.2.res.oracle

src/plugins/wp/tests/wp_acsl/oracle_qualif/cnf.res.oracle

...plugins/wp/tests/wp_bts/oracle_qualif/bts_1174.res.oracle

...ugins/wp/tests/wp_bts/oracle_qualif/bts_2471.1.res.oracle

...ugins/wp/tests/wp_bts/oracle_qualif/bts_2471.2.res.oracle

...gins/wp/tests/wp_bts/oracle_qualif/issue_143.3.res.oracle

src/plugins/wp/tests/wp_plugin/fallback.i

src/plugins/wp/tests/wp_plugin/oracle/fallback.res.oracle

...plugins/wp/tests/wp_plugin/oracle_qualif/abs.1.res.oracle

...plugins/wp/tests/wp_plugin/oracle_qualif/abs.2.res.oracle

...ins/wp/tests/wp_plugin/oracle_qualif/convert.1.res.oracle

...gins/wp/tests/wp_plugin/oracle_qualif/fallback.res.oracle