0.1.b2

More comprehensive docstrings.

Also MaskGroup objects:

• can be instantiated from structured numpy arrays
• can be subscripted with numbers (to access all masks at once) as well as strings (to access individual mask)

0.1.b1

Data structures to encapsulate and operate over particle physics data, including:

• Four momenta
• PDG codes
• Colour codes
• Helicity
• Status codes
• Hierarchy

Specifically, the data is embedded on graphs, so the hierarchy is encoded with an COO adjacency list, and utilities are included for converting this to matrix representations, and further manipulating those representations.