Small fixes to field cvs

There was a small bug in the application of forces.  Small change to
ActionAtomistic just moves the if condition checking there are atoms
from the constructor to parseAtomList.
Fixed small bug in field cvs.

This bug only appears when calculations are run in parallel
Added a tool for you to find the global minimum of function on a grid

You find the global average by doing a weighted sum of all the
elements on the grid.  The weights are given e^(-f(x)/2*sigma**2)
where f(x) is the function and sigma is a user specified temperature