EduNLP v0.0.3

1. update formula ast: supporting more symbols and functions defined in katex
2. add tokens to vector tools, including word2vec and doc2vec using gensim
3. sci4sif support tokenization grouped by segments
4. add special tokens: \SIFTag and \SIFSep
5. add item to vector tools
6. add interface for getting pretrained models, where the supported model names can be accessed by edunlp i2v in the command console