[Proposal] rename `dirichlet_tags` to simply `tags`

[fverdugo]

Now:

V = TestFESpace(model,reffe,dirichlet_tags="dirichlet") # <-- note here
Ω = Triangulation(model,tags="domain")
Γ = BoundaryTriangulation(model,tags="neumann")

Proposal:

V = TestFESpace(model,reffe,tags="dirichlet") # <-- note here
Ω = Triangulation(model,tags="domain")
Γ = BoundaryTriangulation(model,tags="neumann")

This would lead to less writing and be more consistent with the other constructors. The change is feasible since we only have (and will possibly always have) one kind of tags in the FESpace constructor. In addition, it would fix https://github.com/gridap/Tutorials/issues/78