Lazy loading of mappings

[stephen-dixon]

• libtokamap throws if there are mappings in a mapping file which use an un-registered data source at runtime, even if you request another mapping in the same file that doesn't require that data source
• libtokamap throws if there are un-registered data sources used in a different group (IDS) from the one being requested

is it possible to change behaviour to only throw when a mapping using an un-registered data_source is explicitly requested (lazy loading).