Fix performance issue with marginalization during prediction
Fix performance issue with marginalization during prediction

Previously, we implicitly included numpy arrays directly into the
tensorflow graph during prediction, causing runtime slowdowns due to the
growing graph size.  This commit makes use of a placeholder variable
instead, and extracts the set of ops outside the inner loop.

Closes #21.

dusenberrymw/deep-histopath,dusenberrymw/deep-histopath