Add shorter methods for creating shell and func-processes, with auto-generated names.

[samuell]

Right now, to create a new process, you have to give it a name, which sometimes takes focus away from the actuall shell command. Also, the method name wf.NewProc is a bit technical.

We could add an alternative, wf.Shell(), that just takes the shell command as parameter, and auto-generates a name, to make simpler workflows more fluent to write. One could still give a name with wf.Name = "some-name", and we could retain wf.NewProc() for compatibility.

So, this:

proc := wf.NewProc("proc", "echo hi > {o:hi}")
proc.SetOut("hi", "hi.txt")

... would become this:

proc := wf.Shell("echo hi > {o:hi}")
proc.SetOut("hi", "hi.txt")

Together with the suggestion in #123, the total would be:

proc := wf.Shell("echo hi > {o:hi:hi.txt}")

We would also then probably do something similar for inline-Go-function based processes:

proc := wf.Func(
    func(t *scipipe.Task) {
        // Function implementation here ...
    },
    []string{"in1", "in2"}, // Specifying inputs
    []string{"out1", "out2"}, // Specifying outputs
    []string{"param1", "param2"} // Specifying params
)