Updates to various places of the documentation.
Test lac/SchurMatrix
Test lac/SchurMatrix; Indented
Merge pull request #165 from dakshinai/test_schur_matrix

Add tests for class SchurMatrix
Adjust based on Guido's feedback.
Merge pull request #164 from bangerth/doc-fixes-8

Updates to the documentation of the quadrature formulas.
Add a function that allows us to query a colored set of cells.

Here is a more complete description of what is happening:

1/ This function is called on a separate task upon calling distribute_dofs() to run
in the background for when we need the result at a later time.

2/ This also introduces new member functions that require us to adjust
the memory statistics.

The new objects increase memory use by DoFHandler by about 1/7th for Q(1)
elements and likely much less for more interesting elements (because the
memory consumption of the set of colored cells only depends on the number
of cells, not on the number of DoFs per cell. On the other hand, memory
consumption of the DoFHandler is not a large fraction of overall memory
use in programs. For example, for a program derived from step-6 (namely,
tests/deal.II/memory_consumption_01.cc), we have these statistics after
the change:

DEAL::Cycle 3:
DEAL::   Number of active cells:       3248
DEAL::   Number of degrees of freedom: 32433
DEAL::Memory consumption -- Triangulation: 1375395
DEAL::Memory consumption -- DoFHandler:    620480
DEAL::Memory consumption -- FE:            12280
DEAL::Memory consumption -- Constraints:   1307806
DEAL::Memory consumption -- Sparsity:      8321760
DEAL::Memory consumption -- Matrix:        16383928
DEAL::Memory consumption -- Solution:      259552
DEAL::Memory consumption -- Rhs:           259552
DEAL::Memory consumption -- DataOut:       1159725

In other words, the DoFHandler is a very small fraction of the matrix,
for example, and this fraction only becomes smaller for more complex
problems with more entries per row in the matrix.

3/ To be usable in realistic situations, this function also needs to
take into account hanging node constraints.