apply_constraints does not need a fully instantiated adaptor

[EddyCMWF]

apply_constraints does not need a fully instantiated adaptor.

At the moment the web portal is instantiating the full adaptor when evaluating the constraints. As the constraints algorithm is the same for all adaptors*, only the abstract adaptor needs to be instantiated. This will reduce the number of web-portal errors whilst developing new adaptors etc, and may reduce any lag caused by instantiating none Abstract adaptors.

• It is possible that we may implement different constraints algorithms, but this would not be on a "per dataset" basis, but more likely on a "per portal" basis. To that end, it may make sense to have the constraints algorithm in this repo instead in the cads-adaptors repo.