Make our own RPC server more capable

[BenediktBurger]

Make the RPC Server more capable in order to replace openrpc as a dependency, which is notorious in breaking PyLECO, see #90 .

• enhance method discovery
  • parameters and return value
• more tests (maybe using the OpenRPC sandbox for generating test files)

Independent PRs

• Fix rpc server to raise an exception if a method is redefined #136