Adding experimental optimisation with gurobi (dependency needed) combinatorial optimization

[willu47]

IN PROGRESS - do not merge

• Refactors morris_util
• Add groups to brute force error solving issue Morris groups with optimal trajectories not correctly computing distance matrix #48

[jdherman]

Awesome! If this is a similar optimization approach to Campolongo et al 07, feel free to merge it into master when you're done.

We'll have to figure out how to do optional dependencies. I'm sure there's a way.

[coveralls]

Coverage increased (+0.97%) to 87.24% when pulling 08a73b9 on willu47:feat_gurobi into 742c1e9 on jdherman:experimental-methods.

[coveralls]

Coverage increased (+0.97%) to 87.24% when pulling 7d426e5 on willu47:feat_gurobi into 742c1e9 on jdherman:experimental-methods.

[coveralls]

Coverage increased (+0.97%) to 87.24% when pulling dc7d53b on willu47:feat_gurobi into 742c1e9 on jdherman:experimental-methods.

[coveralls]

Coverage increased (+0.97%) to 87.24% when pulling dc7d53b on willu47:feat_gurobi into 742c1e9 on jdherman:experimental-methods.

[willu47]

Superseded by #51, which merges into master, rather than jdherman:experimental_methods