Placeholder mechanism for name translation

[jm-glowienke]

• Change preprocessing such that instead of the name a placeholder is inserted indicating the position of the name in question
• post-processing the placeholder is used to replace the placeholder by the variable

[jm-glowienke]

Use pointer-generator architecture: https://github.com/pytorch/fairseq/blob/master/examples/pointer_generator/pointer_generator_src/transformer_pg.py

Idea is from http://www.abigailsee.com/2017/04/16/taming-rnns-for-better-summarization.html

https://arxiv.org/pdf/1506.03134.pdf

[jm-glowienke]

Copy with placeholders already done implicitly, moving pointer-generator approach to new issue #39