Rebuild class hierarchy

[jindrahelcl]

As discussed on the meeting, main issues are:

• [x] Adapt @tensor annotations (introduced in #191)
• [x] Base class for object with hidden_states and encoded and rename these attributes
• [x] Create a classes for sequences and unify the sequence encoders to work with this class.
• [x] Derive the attention classes from ModelPart
• [x] Enable ModelPart to know the feedables it depends on
• [ ] Finally, ModelPart will keep collections of trainable, regularizable, and model variables, possibly summaries

[kocmitom]

Do not forget, that currently the saving of model parts do not work properly. Viz line 54 in model_part, there shouldn't be GLOBAL but MODEL