BIG CHANGES:
- Unified CFG and PCFG into a single file called grammar.
- Significantly simplified the structure of the grammar.
- Verified the correctness of the inside-outside algorithm.
- Verified the correctness of max-marginal parsing.

TODO:
- Enforce gold tags softly, so that parsing does not fail.
- Find a better solution for handling unknown words/tags.
- Optimize max-marginal parsing: currently about x6 slower than CKY.