Implementing COO graph functions

[bkj]

• Implement get_number_of_neighbors, get_starting_edge and get_edge for COO
• Implement examples/test_coo for spot-checking these functions.

I think the implementations are correct, but worth someone double checking.

@neoblizz One thing:
In graph/detail/build.hxx I had to change

using graph_type = graph::graph_t<space, vertex_t, edge_t, weight_t, csr_v_t, csc_v_t, coo_v_t>;

to

using graph_type = graph::graph_t<space, vertex_t, edge_t, weight_t, coo_v_t>;

otherwise it complains about

error: class "gunrock::graph::empty_csr_t" has no member "get_source_vertex"

I think this means you might need to modify how the inheritance works in graph/graph.hxx but I'll leave that to you.

Do not merge this until that view issue is fixed, otherwise it'll break stuff.

[neoblizz]

• Implement examples/test_coo for spot-checking these functions.

I have moved all the unit tests to https://github.com/gunrock/essentials/tree/master/unittests instead of the examples directory. It also supports cmake now, please use that instead.

And merge all the PRs to dev instead, I am changing the base of this one as well.

[neoblizz]

Do not merge this until that view issue is fixed, otherwise it'll break stuff.

I will look at the empty_t problem, I thought I had already addressed it.