Takes a filename in the workspace and runs it as Groovy source text.

The loaded file can contain statements at top level or just load and run a closure. For example:

    def flow
    node('slave') {
        flow = load 'flow.groovy'
        flow.functionA()
    }
    flow.functionB()
    

Where flow.groovy defines functionA and functionB functions (among others) before ending with return this;