Document that the Lagrange Local Basis on Pyramids is only piecewise linear/quadratic

See the thread starting here. In particular we should reference the paper mentioned here.

Following some references in the mentioned publication and the reference given in the pyramidp2localbasis.hh implementation, the two articles, the implementation is based on, could be found:

• pyramidp1localbasis: C. Wieners, Conforming discretizations on tetrahedrons, pyramids, prisms and hexahedrons (1997)
• pyramidp2localbasis: L. Liu et.al. On Higher Order Pyramidal Finite Elements, (2011)

These are symmetric piecewise trilinear/triquadratic basis functions with the following properties:

• Nodal basis functions
• Each basis function is bilinear/biquadratic on the rectangular face
• Each basis function is linear/quadratic on all triangular faces
• Each basis function is continuous in the interelement boundary

For the numerical quadrature one could use a composite quadrature rule, composed of two tetrahedral parts.

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