Support numpy-style interface for methods and factory functions.

[tglarner]

The discussion in pull request #10 lead to this issue: The API should support method and function calls in the style of numpy, i.e.

def op(self, (d1, d2, ,...), *args):
    ...

and

def op((d1, d2, ...), *args):
    ...

This is different to the current API, which looks like

def op(self, *datasets):
    ....

and

def op(*datasets):
    ...

respectively.

[tglarner]

PR #12 adresses this issue for concatenate by allowing both styles.