rename sfepy/mechanics/contact_planes.py -> sfepy/mechanics/contact_bodies.py
new sfepy/linalg/check_derivatives.py - new check_fx()
use max-norm in check_fx()
new check_vfvx()
new ContactSphere
fix empty array condition in ContactPlaneTerm.smooth_f()
new ContactPlaneTerm._get_force_pars(), update .get_fargs()
new ContactSphereTerm (dw_contact_sphere)
new examples/linear_elasticity/elastic_contact_sphere.py + test