Add generated M^2 files

[pavlo-kuchmiichuk]

This PR contains a couple of changes:

• adds means to create a new data representation annotated-source-sentences - these are the split source sentences with annotations from the source document present;
• adds scripts to automatically create M^2 files necessary for evaluation.

M^2 files derive from the existing data representations, thus if the annotations/tokenization/sentence splitting is faulty, there might be mistakes in M^2 representation as well. Note that currently, running m2-scorer on tokenized-target-sentences as system output does not produce perfect scores, although it should. This should be fixed in future iterations.

[pavlo-kuchmiichuk]

Here are the data points where the number of sentences still differs in source-sentences and target-sentences:

train:

• 0058.src.txt
• 0634.src.txt
• 0801.src.txt
• 0807.src.txt
• 0870.src.txt
• 0882.src.txt
• 0913.src.txt
• 1237.src.txt
• 1245.src.txt
• 1529.src.txt
• 1668.src.txt
• 1849.src.txt

test:

• 0303.src.txt
• 0426.src.txt