The solution is rather slow, but working. The latter is important for
the large tests CI.

[Alg] Added a  class for piece wise linear curve

[LIE/HM] Supporting deactivation by material IDs

Ehlers Single surface yield function material model.

This is based on OGS-5 code. Initial copy by
Xing-Yuan Miao.

[LIE/HM] compute & output second varaibles

[Mat] Fix unknown xml/html tag warnings.

Doxygen documentation2

because the opening formula command is wrong.