Adds an l2 (little ell) norm function to invmed_tree.txx and hpp. This norm is not one for the function f(x)=1 on the unit cube, but rather it depends on the number of places we use to approximate the function. We could divide by the number of points to normalize, maybe this is a good idea, maybe it isn't. Not sure. This commit also updates test.cpp to make use of this new function.