Agglomerate using connected components

[fdrmrc]

To be merged after #129. This PR expands the interface to define agglomerates by checking if the input patch of cells is connected. If not, the connected components are computed using depth-first-search on the graph associated to the agglomerate and each one is agglomerated in the standard way. The relevant commit is aed376b

The following test explains the matter and has been added. Consider the following fine grid and flag for agglomeration the following cells:

$K_0 = \{ 0,1,2,3,12,13,14,15\}$
$K_1 = \{4, 5, 6, 7, 8, 9, 10\}$
$K_2 =\{11\}$

x - - - - - - - - - - - - -x
|     |     |      |       |
|  10 |  11 |  14  |  15   |
|     |     |      |       |
| - - | - - - - - -| - - - |
|     |     |      |       |
|  8  | 9   |  12  |  13   |
| - - | - - - - - -| - - - |
|     |     |      |       |
|  2  |  3  |  6   |   7   |
| - - | - - - - - -| - - - |
|  0  |  1  |  4   |    5  |
|     |     |      |       |
x - - - - - - - - - - - - -x

$K_0$ and $K_1$ are clearly disconnected. The new routine produces the following final set of agglomerates, by agglomerating the connected components separately.

$K_0 = \{0,1,2,3\}$
$K_1 = \{4, 5, 6, 7\}$
$K_2 = \{11\}$
$K_3 = \{8, 9, 10\}$
$K_4 = \{12,13,14,15\}$

The test simply checks that we can capture exactly on this new mesh some manufactured solutions which live in the test space.

[fdrmrc]

The failing test will be fixed in #134