Consider removing "U" from part 1 tutorial

[gregtucker]

An observer pointed out that the baselevel/"uplift" term in the hillslope evolution exercise may be confusing to non-geomorphologists. In fact, it isn't necessary to the purpose of that particular lesson, and could be removed from the derivation. For the steady analytical solution, one could instead set the left-hand side (dz/dt) to a constant erosion rate -E, and the solution is identical. Then, for the numerical solution, one could lower the elevation of the two boundary nodes instead of raising the interior nodes.