Would be great to be able to do more sophisticated stuff with out configuration files - so for example, use math functions on inputs, use (e.g. xboa) units. At the moment we can't import modules because imported modules can't be converted to json. Need a new function that excludes data that cannot be converted to json from the input (and makes a warning message I guess).

Query: do we only want to allow certain types to do this e.g. class, module, namespace (but explicitly not instances of classes). Saves johnny user from getting confused when his xboa.Hit fails to import...

Implemented the general case in rogers. Needs tests.