Update HunFlair tutorial to Flair 0.12

[alanakbik]

This PR updates the HunFlair tutorial to the changed syntax in Flair 0.12:

• there is now a separate module for sentence splitting, changing the import statement (from flair.splitter import SciSpacySentenceSplitter instead of from flair.tokenization import SciSpacySentenceSplitter )
• the tagger is now implemented as a MultitaskModel, and all taggers are now initialized using Classifier by default, meaning you init and print labels like this:

from flair.data import Sentence
from flair.nn import Classifier

# make a sentence 
sentence = Sentence("Behavioral abnormalities in the Fmr1 KO2 Mouse Model of Fragile X Syndrome")

# load biomedical tagger
tagger = Classifier.load("hunflair")

# tag sentence
tagger.predict(sentence)

# print labels
for entity in sentence.get_labels():
    print(entity)

Open Issue: SciSpacy instructions

However, I am unable to get the scispacy tokenizer to run - I think this is because Flair now requires Python 3.7.

@mariosaenger @leonweber @piamarlene @sg-wbi @WangXII: could one of you check how the pip install instructions need to be changed so that the SciSpacy example works? The current instructions are as follows and no longer work:

Current install instructions:

pip install scispacy==0.2.5
pip install https://s3-us-west-2.amazonaws.com/ai2-s2-scispacy/releases/v0.2.5/en_core_sci_sm-0.2.5.tar.gz

Current tag instructions:

from flair.data import Sentence
from flair.nn import Classifier
from flair.tokenization import SciSpacyTokenizer

# make a sentence and tokenize with SciSpaCy
sentence = Sentence("Behavioral abnormalities in the Fmr1 KO2 Mouse Model of Fragile X Syndrome",
                    use_tokenizer=SciSpacyTokenizer())

# load biomedical tagger
tagger = Classifier.load("hunflair")

# tag sentence
tagger.predict(sentence)

[alanakbik]

@mariosaenger @leonweber @piamarlene @sg-wbi @WangXII merging this now for the hotfix release, let me know if you figure the issue with scispacy out.