Draft: Separate the stokes assembling from the definition of the bases
In order to have a uniform implementation of multiple different stable Stokes bases, I want to separate the assembling code from the definition of the basis. This is extracted from !435 that introduces bases with bubble functions to define, for example, a MINI element or a P2b+P1^disc
basis.
ToDo
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Check that the documentation of the Stokes example can be generated properly.