The ability to turn DOFs "on" and "off"

[djinpatch]

Being able to target specific dofs is sometimes useful during optimization. This allows for only optimizing for current, only optimizing for coil shapes, or only optimizing specific coil shapes. Here is an example of how this is done in simsopt:

if current_opt == True: # only target current
        # fix all geometric dofs 
        for c in coils_opt:
            c.curve.fix_all()

.
.
.

# unfix all geometric dofs 
        for c in coils_opt:
            c.unfix_all()
            
            if ellipse_only == True: # only let specific dofs optimize       
                for c in  bs_opt.coils:
                    c.curve.fix_all()

                    # allow coil to slide on z -axis 
                    c.curve.unfix(f'zc(0)') 

                    # allow coil to make an ellipse in the xy plane
                    c.curve.unfix(f'xc(1)') 
                    c.curve.unfix(f'ys(1)') 

                    # allow coil to bend in z plane 
                    c.curve.unfix(f'zc(2)')