Improve and add unittest for [d]zoom optics

This PR

• Move the logic related to this optic into specific methods
• Implement the inherited move_magnification
• Update exposed target magnification
• Update the final magnification (after a move) from motor locations
• Added unittest (using config from BM18)

The tests show that there is a huge discrepancy between requested magnification, and resulting magnification (requested mag -> move motor -> motor pos -> computed mag). I can't tell if it is normal or not.