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

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

Proposal:
```julia
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

Search the Gridap.jl source for the TestFESpace constructor and all uses of `dirichlet_tags`. Check the related constructor examples and any tests that cover these keyword arguments; done means the shorter `tags` form is accepted consistently and the old naming is no longer required.

Written by the indexing model from the issue text.