Think about using global std::vector objects in the value_list

Think about using global std::vector objects in the value_list
function, in order to avoid repeated memory allocation for these
objects. However, no way springs to mind as apparent, basically
because the length of these vectors depends on the number of points at
which the derivative is to be computed, which may vary between
successive calls to the function and we don't want to grow the vectors
beyond all bounds if once many points are required and usually only a
few.

Nevertheless, clean up some things...

git-svn-id: https://svn.dealii.org/trunk@4884 0785d39b-7218-0410-832d-ea1e28bc413d