modified the documentation of the Arpack solver
corrected Arpack section in Step-36
added a step-36 test to arpack category
Improved comparison operators for TriaAccessorBase

The previous commit was a bit too aggressive because it is still useful to catch the case when comparing iterators to two different triangulations (leading to infinite loops).

Therefore, I now merely extended the assertion to not trigger when one of the two objects is default constructed, i.e., does contain a null pointer to tria. The comparison then does not need tria == a.tria because invalid iterators use invalid cell and level index, too (leading to !=).
Initialization of FEEvaluation like FEValues

A new way to initialize an FEEvaluation object (fast tensor product evaluation routines) has been introduced.
It takes the same arguments as the initialization method of FEValues, namely a Mapping, a FiniteElement, a
one-dimensional Quadrature<1> object, and update flags. This enables using FEEvaluation on the fly just as
FEValues. Since FEEvaluation treats objects of one base element only, this class can select which components
to read.

The concept is explained in the documentation of the FEEvaluation class and an example for the Laplace matrix
is given. However, it also works for the Stokes operator as shown by the test program assemble_matrix_02.
Merge pull request #258 from kronbichler/matrix_free_assembly

Initialization of FEEvaluation like FEValues
Merge pull request #263 from davydden/arpack

Arpack