Added checking for NaN

WATER injections ( all the cells were not tracked at all points in
time, you need to ignore the NaN values and track them at the points of
time you can get them at..this is different, PAY ATTENTION to the fact
that not all cells are tracked over all points of time and that the
data structure containing the vertices contains different values for
different points of time
Derivative
subtraction / division (normalization)