Thereby documentation is found better in the source code.

Script already contains a usage comment.

Create OGS-5 direct Neumann boundary conditions (in OGS-5: source terms)

Cleanup some memory leaks and division by zero.

auto const darcy_v = (EigenExpr).eval() returns a temporary
object. (Eigen 3.2.9).
Error is detected by clang's address sanitizer.
AddressSanitizer: heap-use-after-free on address ...

Also slightly clean up the surroundings.

Make use of named functions to compute secondary variables

Updated vtkdiff

Correction needed because the check in vtkdiff has been fixed

* context: class -> struct
* global vector from named fct: result_cache -> result
* cached 2ndary var: fixed member name