[Fluid] Move BlockSize to data containers

[rubenzorrilla]

📝 Description
In this PR I'm moving the BlockSize static constexpr variable from the FluidElement base class to the data containers. This will make possible to use current element inheritance structure with formulations featuring unknowns different to u-p (e.g., thermally coupled ones). Also note that I took the chance to move the data containers from custom_utilities folder to the data_containers one together to the element.

@jgonzalezusua note that you had an error in the block size definition in your DEM coupled element. As far is I see you were not using that one but the one from the base element. In any case, take this observation into account just in case.

@uxuech note that this will affect your new elements when merging the master (you need to do the corresponding changes mimicking the ones in here in your branch).

[jgonzalezusua]

Thanks for fixing the error, it was just a small mistake. I did not used that variable.

[rubenzorrilla]

Thanks for fixing the error, it was just a small mistake. I did not used that variable.

Yes. It was silently there, the point is that with this change it woke up 😄 . Nevertheless, just for you to know.

[rubenzorrilla]

Note that last bunch of commits only remove the redundant custom_elements/ from the includes.