generalization of nodes that receive data via exogenous callback

[straszheim]

Seems that the pattern

ecto_ros_subscriber<InputType, OutputType, ConverterType>

might be further generalizable... if I'm a JACK audio client then I also register a callback and spin. Maybe better,

AsyncInput<InputType, OutputType, ConverterType, SubscribeAndSpinType>

e.g. something like

AsyncInput<common_msgs::Image, cv::Mat, Img2MatConverter, ROSSubscriber>

[ethanrublee]

Moving to Acrasidae

[straszheim]

also, the ros stuff (as discussed on IRC) could probably be sped up / ensmallified by having one cell type for pub/sub/bag, and an internal impl/registry that maps something to the instance's message type