In an internal function called from a virtual function, do not call virtual functions.

[bangerth]

Finding this bug took probably on the order of half a week of actual work, for the addition of 34 characters. My many failed attempts at figuring out what is going on are chronicled in geodynamics/aspect#5421 and geodynamics/aspect#5577 .

The root cause here is that in ASPECT, we derive a class from SphericalManifold in which we create a sphere with topography (the Earth) by stacking operations in the same way as we often do with manifolds: You just concatenate operations to form the pull back and push forward operations. As a consequence, the overriding functions then call back into the functions in the base class. Then, our implementation of functions in SphericalManifold calls internal functions that do certain things and, in at least one case, further delegate to yet another function -- which is virtual! So we end back up in user space, apply our own secondary transformation one more time, and then call back into the SphericalManifold class. This is not desirable. The internal function only calls the other function for convenience, and so it needs to call the one in SphericalManifold, not the one in user space.

This patch fixes this. I cannot express how relieved I am to have fixed this.

(For the purposes of cross linking: We have had trouble with this class before. See #16242 and what is linked from there.)

[bangerth]

@gassmoeller FYI

[kronbichler]

The failing tests are addressed by #16740, so let us merge this.

[gassmoeller]

Wow, thanks for figuring this out! The wonders of polymorphism I guess :-)