Extend FEFunctions to behave like lin alg vectors

Perhaps we can add new methods to our lin alg interfaces

https://github.com/gridap/Gridap.jl/blob/master/src/Algebra/AlgebraInterfaces.jl

so that `FEFunctions` behave like linear algebra vectors

The functions to extend include `fill_entries!` , `add_entries!` , and `scale_entries!`, which should modify both free and dirichlet values.

I also would overload `wh = similar(uh)` so that we can implement
```julia
function add_entries(b,c)
a = similar(b)
add_entries!(a,b,c)
a
end
```

Idem for `scale_entries`.