[StructMech] make primal element member private in FD adjoint elements

[armingeiser]

A general comment to adjoint elements/conditions which does not belong directly to this PR but In my eyes worth to mention: During my review I observed a nonuniform usage of the pointer of the primal element. Sometimes the member variable is directly used, sometimes the function pGetPrimalElement is used. And I don't understand why the function pGetPrimalElement is needed since the pointer to the primal element/condition is a protected member variable. Of course this is only a minor detail and not part of this PR but I was just wondering.

If you make the pointer a private member then you can only access it via the Get-Function
This could also avoid unintended uses if you want to make some restrictions

Originally posted by @philbucher in #4338 (comment)

[philbucher]

is this solved?