8.1.0

Added: ElementTriHHJ0 and ElementTriHHJ1 matrix finite elements for implementing the Hellan-Hermann-Johnson mixed method (see ex48.py)
Added: ElementHexSkeleton0, piecewise constant element on the skeleton of a hexahedral mesh
Added: ElementHexC1, C1-continuous hexahedral element
Added: Mesh.restrict now preserves subdomains and boundaries
Added: skfem.utils.mpc for setting multipoint constraints
Added: Basis.get_dofs now supports fetching DOFs at specific nodes through kwarg nodes
Added: DofsView.sort for sorting a set of DOFs returned by Basis.get_dofs
Added: CellBasis.with_elements as a shortcut for initiating a new Basis with subset of elements
Added: Mesh.refined now preserves subdomains in adaptive mesh refinement
Fixed: Mesh.refined used to modify the element connectivity of the original mesh