Automatic Transformer ordering

[mrossinek]

What should we add?

Now that we are adding more transformers (e.g. #640) the ordering in which multiple transformers are applied to a problem becomes relevant. In fact, some of these may require to be executed last or may even be incompatible with other transformers (#48 in the future).

In order to make the life of users easier, we should implement some sort of "priority" label according to which multiple transformers can be ordered and possible even made mutually exclusive.

[mrossinek]

I am closing this issue because as of Qiskit Nature v0.5.0, transformers are applied by users manually.
It is the responsibility of the user to do so in the correct and sensible order.

We should ensure that our documentation reflects potential mutual exclusivity and/or order dependence.