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.