Use of functools.lru cache makes scifit-fem objects un-pickle-able

Use of functools.lru cache makes scifit-fem objects un-pickle-able: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). https://github.com/kinnala/scikit-fem/blob/bc57d968e56e6b89a99e35eac26ef7bc81b7a46a/skfem/mapping/mapping isoparametric.py L64