This aims to be a first tentative approach at avoiding the need for manual joining when filtering a table.

Before the filtering itself relationship of the models are retrieved. If one of the filtering field is associated with a relationship and it contains values not only null values the corresponding table is joined. This accommodates for nested filters and tables connected through a secondary table.

I call this change tentative for three main reasons:

1. I am not sure this is something of only personal interest or if there is a need for it
2. I do not have experience working with MongoDB through mongoengine, as such I still have not touched the code relative to it.
3. Some tests are failing due to warning to cartesian products between columns. In my use case this does not seem to generate wrong results, but I am not 100% sure on the best course of action could be in this case to avoid the warning altogether, as such I have not changed the tests yet to pass. Some are still failing due to the aforementioned warning.