Enable generation of coarse meshes through R-tree

[fdrmrc]

This PR introduces the visitor required to build coarse meshes out of finer one by using the R-tree. Its implementation is in include/rtree_visitor.h. The function PolyUtils::extract_children_of_level(rtree,level) returns the required information.

[fdrmrc]

I'll add a test tomorrow. This PR will also fix the CI error on master (this function is missing).

[fdrmrc]

I've added a test. It starts with a hyper_cube with 1024 elements and uses the R-tree to produce uniform partitions (see hidden output)

Extraction level = 1
Size of fine triangulation: 1024
Total number of available levels: 4
N subdomains = 4
256 cells have subdomain id = 0
256 cells have subdomain id = 1
256 cells have subdomain id = 2
256 cells have subdomain id = 3

Extraction level = 2
Size of fine triangulation: 1024
Total number of available levels: 4
N subdomains = 16
64 cells have subdomain id = 0
64 cells have subdomain id = 1
64 cells have subdomain id = 2
64 cells have subdomain id = 3
64 cells have subdomain id = 4
64 cells have subdomain id = 5
64 cells have subdomain id = 6
64 cells have subdomain id = 7
64 cells have subdomain id = 8
64 cells have subdomain id = 9
64 cells have subdomain id = 10
64 cells have subdomain id = 11
64 cells have subdomain id = 12
64 cells have subdomain id = 13
64 cells have subdomain id = 14
64 cells have subdomain id = 15

Extraction level = 3
Size of fine triangulation: 1024
Total number of available levels: 4
N subdomains = 64
16 cells have subdomain id = 0
16 cells have subdomain id = 1
16 cells have subdomain id = 2
16 cells have subdomain id = 3
16 cells have subdomain id = 4
16 cells have subdomain id = 5
16 cells have subdomain id = 6
16 cells have subdomain id = 7
16 cells have subdomain id = 8
16 cells have subdomain id = 9
16 cells have subdomain id = 10
16 cells have subdomain id = 11
16 cells have subdomain id = 12
16 cells have subdomain id = 13
16 cells have subdomain id = 14
16 cells have subdomain id = 15
16 cells have subdomain id = 16
16 cells have subdomain id = 17
16 cells have subdomain id = 18
16 cells have subdomain id = 19
16 cells have subdomain id = 20
16 cells have subdomain id = 21
16 cells have subdomain id = 22
16 cells have subdomain id = 23
16 cells have subdomain id = 24
16 cells have subdomain id = 25
16 cells have subdomain id = 26
16 cells have subdomain id = 27
16 cells have subdomain id = 28
16 cells have subdomain id = 29
16 cells have subdomain id = 30
16 cells have subdomain id = 31
16 cells have subdomain id = 32
16 cells have subdomain id = 33
16 cells have subdomain id = 34
16 cells have subdomain id = 35
16 cells have subdomain id = 36
16 cells have subdomain id = 37
16 cells have subdomain id = 38
16 cells have subdomain id = 39
16 cells have subdomain id = 40
16 cells have subdomain id = 41
16 cells have subdomain id = 42
16 cells have subdomain id = 43
16 cells have subdomain id = 44
16 cells have subdomain id = 45
16 cells have subdomain id = 46
16 cells have subdomain id = 47
16 cells have subdomain id = 48
16 cells have subdomain id = 49
16 cells have subdomain id = 50
16 cells have subdomain id = 51
16 cells have subdomain id = 52
16 cells have subdomain id = 53
16 cells have subdomain id = 54
16 cells have subdomain id = 55
16 cells have subdomain id = 56
16 cells have subdomain id = 57
16 cells have subdomain id = 58
16 cells have subdomain id = 59
16 cells have subdomain id = 60
16 cells have subdomain id = 61
16 cells have subdomain id = 62
16 cells have subdomain id = 63