Lark parser

[henryiii]

This replaces the decay backend with the Lark parser, making it easier to maintain. While there is no front-end yet, a decfile parser is also included. A few features have come along:

• Particle: now supports more types of input in the renamed from_string (was from_AmpGen)
• Particle: now has a user command line program python -m decaylanguage.particle <particles>
• Particle: Cleaner repr
• Decay: Now refactored with a base class (for future DecFile parser possibly)

Along with one regression:

• Cartesian lines are not combined yet (needs better test or should be dropped from supported language)

And several bugs fixed, primarily with the decaylanguage script (which will probably be moved to decaylanguage.decay at some point, since decaylanguage.particle and decaylanguage.decfile are distinct and equal)

[coveralls]

Coverage decreased (-25.3%) to 55.251% when pulling 0c4258b on lark into d6be14b on master.