New fsl_generate_module_manifest command

For generating/updating a JSON manifest file. Some other enhancements too:

• Added ability to refer to 'built-in' manifests by aliases - the default manifest is referred to as fslcourse, and the Graduate Programme manifest as gradcourse, e.g. fsl_add_module -m gradcourse.
• Added support for a size field in manifest entries, specifying the file size in bytes.

fsl/scripts/fsl_add_module.py

@@ -205,14 +205,15 @@ def parseArgs(argv : List[str]) -> argparse.Namespace:
         args.category = None
 
     # Built-in manifest referred to by alias, e.g. "fslcourse"?
-    if not (args.manifest.startswith('http') or op.exists(args.manifest)):
+    if not (args.manifest is None            or
+            args.manifest.startswith('http') or
+            op.exists(args.manifest)):
         try:
             args.manifest = OFFICIAL_MANIFEST_URLS[args.manifest]
         except KeyError:
-            parser.error(
-                f'Unknown manifest: {args.manifest}. Known manifests: ' +
-                ", ".join(OFFICIAL_MANIFEST_URLS.keys()) + ' (or give a URL '
-                'to an alternative manifest file)')
+            warning(f'Unknown manifest: {args.manifest}. Known manifests: ' +
+                    ", ".join(OFFICIAL_MANIFEST_URLS.keys()) + ' (or give a '
+                    'URL to an alternative manifest file)')
 
     if args.archiveDir:
         args.archiveDir = op.abspath(op.expanduser(args.archiveDir))