What is the difference between "RotMat" and "RotMat_s"?
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"RotMat" contains three functions, one for the rotation matrix about each
coordinate axis. E.g., "RotMat.R1(30)" gives you the rotation matrix for
a positive rotation of 30 deg about the 1-axis.

In contrast, "RotMat_s.R1_s()" provides the symbolic rotation about the
1-axis. This is useful if you for example want to calculate the full
Fick-rotation-matrix.
