Fix for gagues issue #190

This pull request has a fix for determining gauges indices given the coordinates, (taking into account that the coordinate values corresponds to the center of the cell not the edge)

This pull request also has modification for testing gauges. 

I assume now setting the coordinates is correct, so issue #190 can be closed.

However, Some gauge tests still does not pass, those are for num of processes =7,14,21...

This seems funny but for these runs the number of time steps taken was
less by 1 time step.. therefore the number of elements in test and expected
gauges arrays does not agree.
I am not sure why the max cfl and hence the time step sizes and number of
time steps is different in this choice of number of processes, while they exactly
match for other choices of num of processes?..