Use numpy.loadtxt instead of newer numpy.genfromtxt in Hemingway driver.

Use numpy.loadtxt instead of newer numpy.genfromtxt in Hemingway driver.

From
  http://docs.scipy.org/doc/numpy/user/basics.io.genfromtxt.html
"In particular, genfromtxt is able to take missing data into account,
when other faster and simpler functions like loadtxt cannot"

We prefer loadtxt because:
  * It's faster and we don't need to handle missing data.
  * It's older (genfromtxt added in Numpy in 2009/01/19, released in
    v1.3.0), so it's more likely to be in the user's stock Numpy
    installation.