Fix "warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false [-Wstrict-overflow]"

This was a tricky one. It was highly non-obvious why the compiler even ran into this issue. My best guess is as follows: The compiler was trying to optimize the loop over the vertices in testStaticRefinementGeometry(), test-refinement.cc:122. To do this it needed to statically evaluate vEnd(refinement), where refinement is unsigned, but converted to int due to the signature of vEnd(). The constructed iterator stores it as unsigned _level, and it also stores the current position as unsigned _index. So there is a conversion from unsigned to int and back to unsigned.

Changing the parameter and return value of vEnd() to unsigned turns out not to be enough: vEnd calls nVertices(), which also takes and returns an int. Changing that to unsigned too is still not enough, because nVertices evaluates the expression Power<dimension>::eval((1<<level)+1). While level is now unsigned, the literal 1's are not, and the result of operator<<() will have the type of the (promoted) left operand (the "usual integral conversions" do not happen for <<). Thus we need to change the literals to 1u.

To be clear: the warning happens because of conversion from unsigned to int and then back to unsigned, where the compiler is unable to fully track all possible values and cannot prove that everything is fine.

This patch fixes this by making the level parameter unsigned, and by making sure that any literals are suffixed by u.

Adresses #5 (closed).