Improve datamodel unitary tests for SWMM

[tproduit]

In https://github.com/QGEP/datamodel/blob/master/test/test_swmm.py, the tests are based on the number of object expected in the views. Once that the SWMM export will be more mature, the tests should rely on the number of objects in the main tables.

Exemple: If there are 5095 reaches in qgep_od.reach, there should be 5095 conduits in SWMM.

However, other relations might be less straightforward.