Add mesh database infrastructure

The database is just populated with a small 3d cartesian grid which is
still inaccessible.

This is just a proof of concept: the mesh is build for both `unit_tests`
and `mpi_unit_tests`.

The optimization resulting from the previous is already noticed even
with Release build type.

Using `ctest` (old `make test`) one gets the following execution times
- without mesh building: 3s
- with a simple mesh building: 31s

Using the new `test` target one gets the following execution times
- without mesh building: 1.3s
- with a simple mesh building: 2.7s

(the `make test` results are actually obtained using `ninja` since the
test is so fast that the majority of the spent time using `make` is
spent computing target dependencies)

Using `make test` with the new `test` target one gets the following
execution times
- without mesh building: 1.95s
- with a simple mesh building: 3.45s