Improve run time of identifySubdomainMesh

The run time was improved due to:

• selection of better (smaller) ratio of number of mesh nodes per grid cell - this leads to lower number of distance computations / comparisons
• pre-computing the connected elements to nodes and use it multiple times
• use unordered_map instead of map
• make OpenMP parallelization available

The times given in the table are without reading / writing the meshes. The first number in the sum is the time for the MeshNodeSearcher construction, the second number is the time for the identification of nodes/elements.

OMP_NUM_THREADS	master	MR
1	0.50 + 536.76	0.78 + 142.58
2		0.75 + 80.23
4		0.74 + 55.36
8		0.76 + 43.05

1. Feature description was added to the changelog
2. Tests covering your feature were added? - identifySubdomains tests passes in CI.
3. Any new feature or behaviour change was documented?