Center signal wrt to OY axis

[tanyapole]

Module ECG.api
Currently the method convert_image_to_signal does not center signal wrt to OY axis.
This issue doesn't affect methods, that use relative signal values (check_ST_elevation, evaluate_risk_markers, diagnose_with_STEMI).
However this issue can affect methods, that use absolute signal values (diagnose_with_NN)

[tanyapole]

Useful materials for fixing the issue:

• Sedaaghi, Mohammad & Khosravy, Mahdi. (2003). Morphological ECG Signal Pre-processing with More Efficient Baseline Drift Removal. Proceedings of the IASTED International Conference on Artificial Intelligence and Soft Computing. 7.
  researchgate