function register (entity) {
    registerEntity(entity)
    var component = entity.component
    if (component.registered) return

    // initialize sources once for a component type.
    registerSources(entity)
    component.registered = true
  }
Register an entity.

This is mostly to pre-preprocess component properties and values chains.

The end result is for every component that gets mounted,
you create a set of IO nodes in the network from the `value` definitions.

@param {Component} component