Add Fx and Fy as outputs on cylindrical joints

[kristsaet]

As heading, missing as pr. now.

[kmokstad]

The forces (or any other solution variable) for the translational X- and Y-directions for cylindric joints can not be plotted, since these DOFs do not exist for these joints. This is because a cylindric joint constrains the slave to always move along the slider path, and these constraints are enforced through a transformation of the DOFs in which the two translations are explicitly eliminated (see the Theory Guide, Section 6.3 "Multi-master Joints" for details). The same is true for prismatic joints, there this applies to the rotation about the local Z-direction as well. Therefore, this is not easily fixed and I suggest to find other ways to obtain the wanted result quantities (from the slave triad, for instance).