Added get_prover_config in Whyconf.

This is more convenient when using only the Why3 API from SPARK for
example.

src/driver/whyconf.ml

@@ -750,6 +750,8 @@ let save_config config =
 
 let get_main config = config.main
 let get_provers config = config.provers
+let get_prover_config config prover =
+  Mprover.find prover (get_provers config)
 let get_prover_shortcuts config = config.prover_shortcuts
 let get_policies config = config.provers_upgrade_policy
 let get_prover_upgrade_policy config p =

src/driver/whyconf.mli

@@ -127,6 +127,10 @@ val get_provers : config -> config_prover Mprover.t
 (** [get_provers config] get the prover family stored in the Rc file. The
     keys are the unique ids of the prover (argument of the family) *)
 
+val get_prover_config: config -> prover -> config_prover
+(** [get_prover_config config prover] get the prover config as stored in
+ the config. Raise Not_found if the prover does not exists in the config. *)
+
 val set_provers : config ->
   ?shortcuts:prover Mstr.t -> config_prover Mprover.t -> config
 (** [set_provers config provers] replace all the family prover by the