Support 'dense' as a format of scipy sparse that returns a numpy array. The idea is to add support for returning the "dense" format for a number of functions that currently return sparse but don't have a format option. This is based on a discussion in #7973 that includes this comment:

to_scipy_sparse_array is another function that returns a scipy sparse array and has the format= argument, so it also currently supports "dense" format (but this isn't documented).

The other functions I found that return scipy sparse arrays but don't have a way to specify the format are adjacency_matrix, attr_sparse_matrix, bethe_hessian_matrix, incidence_matrix, laplacian_matrix, normalized_laplacian_matrix, and tournament_matrix. I don't recommend adding format= to these functions in this PR, but if anybody thinks doing so is a good idea then please make a new issue or PR to continue exploring this option.