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

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