contracts.UserRegistry.registerUser(..) and similar evocations should work

[jellegerbrandy]

At the moment, contracts.UserRegistry` is the interface constructed from the ABI - so it has no error handling, each function call at most a single transaction, etc.

The idea is that userRegistry.registerUser first does some typechecking fo arguments, and then calls the ABI method on userRegistry.methods.registerUser(...).send() )

Concretely, the approach is that for each contract that is accessible from the Paratii object, there will be a js class that will extend the class created by web3.eth.Contract(ABISPECS).

For a first iteration we can do the minimum required for #1, and just map userRegistry.registerUser(...args) to userRegistry.method.registerUser(...args).send() directly