code review tidy (use Preconditions.checkNotNull)
support for starting children of SoftwareProcess, plus a new VanillaSoftwareProcess making it very easy to create software-based entities, and example tests (for yaml in VanillaSoftwareYamlTest.java and vanilla-software-with-child-blueprint.yaml, though that doesn't yet have children; and with the children in VanillaSoftwareProcessAndChildrenIntegrationTest.java);
also make the YamlLauncher support stopping, and tidies to how StartableMethods works, returning tasks which can be used more flexibly