Refactor embedding logic for data points

[alanakbik]

Refactoring of embedding logic for data points:

• removes the global variable embedding_storage_mode
• moves up set_embedding() and to() from the data point classes (Sentence, Token, etc.) to their parent DataPoint
• allows dynamic embedding names to be passed to store_embeddings for small speed up
• the set_embedding() method no longer transfers embeddings to flair.device (only when getting this is done now)
• DocumentPoolEmbeddings class can now be instantiated only with a single embedding