Break postprocessing the velocity out of the function that computes errors.

Break postprocessing the velocity out of the function that computes errors.

The step-61 program now has a separate function
'compute_postprocessed_velocity' that computes the velocity field as
a postprocessing step after computing the pressure variable. The
resulting velocity field is used in both computing errors and when
creating graphical output.

There is really no functional change, it just rearranges the code
and makes it a bit simpler to read. The only functional simplification
is that we don't compute the velocity at quadrature points from shape
function times coefficient, but use FEValues::get_function_values()
instead.