Add function to get vertices of grid

[Shushman]

The function vertices(::AbstractGrid is added in GridInterpolations.jl to allow the caller to obtain a Vector of size length(grid) where each element is one of the vertices of the grid. The order of vertices is deterministically generated.

E.g.

julia> grid = RectangleGrid([0., 0.5],[0., 0.5, 1.])
GridInterpolations.RectangleGrid with 6 points

julia> vs = vertices(grid)
6-element Array{Array{Float64,1},1}:
 [0.0, 0.0]
 [0.5, 0.0]
 [0.0, 0.5]
 [0.5, 0.5]
 [0.0, 1.0]
 [0.5, 1.0]

[coveralls]

Coverage increased (+0.1%) to 97.059% when pulling 1e9ea4e67903c9c2d557afcd7aac86211357925c on Shushman:feature/vertices into c7fb0515c7827df8861f564c6682ad5344c7c4d1 on sisl:master.

[Shushman]

Do not merge - I need ensure the ordering of vertices is consistent with grid data.

[Shushman]

Based on some simple tests, it seems the order of grid data is consistent with the order of vertices, i.e. the first dimension is the least significant bit. Can someone please confirm?

[mykelk]

Can you add a test?

[Shushman]

I have added a test_vertices_ordering() method that compares the members and ordering of the result of vertices() on a grid with a call to ind2x for each valid vertex index.

[mykelk]

I think the function comment needs to be updated?

[Shushman]

Modified function comment to reflect change. Thanks for catching that @mykelk !