#362 Sample operation initial work

Initial version of Sample() operation implemented and tested for numpy and pandas.
Sample() should randomly select some items from a given datasource and return them.
For numpy and pandas, the returned data are new arrays or dataframes or series.

Generally we don't use `assert` in doctests.  Instead we print things out and the doctester compares what is printed to what if computes.

So I would change this test to 
```Python
>>> len(result)
2
```

You could also actually print result and add a comment `# doctest: +SKIP` to the end of the line to stop the doctester from testing that line.  It would be a helpful example, even if it isn't a test.