Move OCR utils functions to utils.py

In https://gitlab.com/teklia/atr/dan/-/blob/14a54602f2ce89b8732da01a018765e459cec541/dan/ocr/utils.py

• LM_str_to_ind
• LM_ind_to_str

can be moved to https://gitlab.com/teklia/atr/dan/-/blob/14a54602f2ce89b8732da01a018765e459cec541/dan/utils.py

Functions should also be renamed

• LM_str_to_ind should be token_to_ind
• LM_ind_to_str should be ind_to_token