ENH: add command-line parameter --cache-dir

This parameter corresponds to the variable
`TLAPM_CACHE_DIR` of the runtime environment.

This commit includes suggestions from
code review of pull request:
    <#47>

Co-authored-by: Damien Doligez <[email protected]>

Author: johnyf

src/params.ml

@@ -353,6 +353,7 @@ stores fingerprint and auxiliary
 files. This directory can be defined:
 - via the environment variable
   that is read below, or
+- via the command-line option
   `--cache-dir`
 *)
 let cachedir = ref ""
@@ -361,6 +362,7 @@ let () =
         cachedir := Sys.getenv "TLAPM_CACHE_DIR";
     with Not_found ->
         cachedir := ".tlacache"
+let set_tlapm_cache_dir dir = cachedir := dir
 
 
 let keep_going   = ref false

src/params.mli

@@ -90,6 +90,7 @@ val keep_going : bool ref;;   (* FIXME check still used ? *)
 val suppress_all : bool ref;;
 val set_smt_logic : string -> unit;;
 val set_smt_solver : string -> unit;;
+val set_tlapm_cache_dir : string -> unit;;
 val printconfig : bool -> string;;
 val print_config_toolbox : bool -> string;;
 val check_zenon_ver : unit -> unit;;

src/tlapm.ml

@@ -68,6 +68,8 @@ let mkdir_tlaps t =
                 cache directory at the path \
                 `%s`. Please ensure that if \
                 a path is given via the \
+                command-line parameter \
+                `--cache-dir`, or via the \
                 variable `TLAPM_CACHE_DIR` of \
                 the runtime environment, \
                 that a directory at that path \

src/tlapm_args.ml

@@ -212,6 +212,16 @@ let init () =
                "<f> load fingerprints from file <f> (save as usual)";
     "--fpp", Arg.Set fp_deb,
              " print the fingerprints of obligations in toolbox messages";
+    "--cache-dir", Arg.String set_tlapm_cache_dir,
+            "<directory> save auxiliary and \
+             temporary files under <directory>. \
+             Alternatively, this directory \
+             can be defined via the variable \
+             `TLAPM_CACHE_DIR` of the runtime \
+             environment. The command-line \
+             parameter takes precedence over \
+             the environment variable. If neither \
+             is specified, the default value is \".tlacache\".";
   ]
   in
   let opts = Arg.align opts in