Add algorithms for distance-regular graphs

[pddshk]

Namely:

• is_distance_regular
• is_strongly_regular
• intersection_array
• global_parameters

Wikipedia reference

NetworkX reference

[gdalle]

Hi, thank you for this contribution and sorry for the delay!
I think these are useful functions, but before I review in detail, could you maybe:

• choose more specific names for global_parameters and perhaps intersection_array (perhaps prefix them with distance_regular)
• test all the functions you introduce, including on cases where they return false
• whenever possible, avoid math in the docstrings (it doesn't display well in the REPL), replace the double backticks with single backticks and leverage unicode instead

Thanks in advance!