Move axis parameter to modification instead of the modifier

[GrmanRodriguez]

Right now the modifier class has an axis parameter that determines the axis of position and dimension modifications.

This parameter should instead be part of the modification itself, to have a better separation of concerns, increase code cohesion, and make the library easier to use.

For example, in the FixedOffsetModifier class there is a function that shifts the origin of the links and joints in the three dimensions. Since this is a method of the modifier class itself we shouldn't create more modifiers inside it, so it works like this:

original_modifier_axis = modifier.axis # save original modifier axis
modification = Modification()

modifier.axis = Side.X # change it for each axis you want to change

modification.add_position(value_x, absolute)
modifier.modify(modification)

modifier.axis = Side.Y

modification.add_position(value_y, absolute)
modifier.modify(modification)

modifier.axis = Side.Z

modification.add_position(value_z, absolute)
modifier.modify(modification)

modifier.axis = original_modifier_axis # restore original modifier axis

This is how it would look with the axis being in the modification:

modification = Modification()

modification.axis = Side.X 
modification.add_position(value_x, absolute)
modifier.modify(modification)

modification.axis = Side.Y
modification.add_position(value_y, absolute)
modifier.modify(modification)

modification.axis = Side.Z
modification.add_position(value_z, absolute)
modifier.modify(modification)

The code is shorter and makes more intuitive sense.

This change would break existing code however, so I'm open to feedback on the idea @traversaro @CarlottaSartore @AlexAntn

Dod
The axis parameter belongs to the Modification class instead of Modifier