To compute the derivative you need the function value (see my paper).
Therefore, it is computed as the first thing in the evaluateDerivative
method.  However, frequently, the assembler also needs the function value,
and also computes it.  Hence the function value is computed twice.
Since computing the function values takes quite a bit of time
this patch removes that redundancy, at the price of a slightly
more complicated API: if you happen to know the correct function
value when calling evaluateDerivative, you can now hand over the
value.  Then evaluateDerivative uses that value instead of recomputing it.

Short measurements have shown a speed increase between 25% and 45%.

[[Imported from SVN: r8362]]