Require unit test for most basic spatial model

[stijnvanhoey]

The adjustment of the spatial model, see 3a0909f#diff-58601479adea99fc373b6cfb66c6ee42a2f769b4a9b9f65f56b137eec146f2e0 is an important adjustment and extension to the model as it introduces the handling of multiple stratifications at the same time.

I do think this really needs unit testing (cfr. the simplified stratified version to test that part, https://github.com/UGentBiomath/COVID19-Model/blob/master/src/tests/test_model.py#L87-L105) in order to make sure we can further extend this.

@twallema, we can support you with setting up the etst but it would be good to have a kind of 'minimal' version of the SIRstratified -> SIRstratifiedSpatial to setup these tests.

As I understand it well, it will work as long as the stratification influences different parameters? If so, this should be clarified as well for other users.