Graph-ensembles

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* Improved and corrected spark submodule.

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* Created submodule spark for allowing some models to be parallelize computations using spark

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* Added option for faster computation of average nearest neighbour properties by allowing for multiple links between the same nodes.
* Added compression option in to_networkx function.

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* Added likelihood and nearest neighbour properties.
* Revisited API for measures to ensure correct recompute if necessary.

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* Added new option for fitting the stripe model that ensures that the minimum non-zero expected degree is one
* Corrected issue in expected degree calculations

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* Added scale invariant probability functional to all models
* Improved methods for convergence with change in API, xtol now a relative measure
* Added pagerank and trophic depth to the library
* Added methods for graph conversion to networkx
* Added methods for computing the adjacency matrix as a sparse matrix