Ole Klein authored Jul 02, 2019 and Oliver Sander committed Aug 31, 2020

In particular, test the first-order implementations, and test them
for all facet orientations.  This uncovers a few bugs which are
fixed by this patch as well.

(cherry picked from commit 32a02606)

Lisa Julia Nebel authored Apr 02, 2020 and Oliver Sander committed Apr 03, 2020

One of the shape functions of the second-order Lagrange element for
pyramids was implemented wrongly, and the derivative along with it.
This went unnoticed for a long time, until I saw that the element
was not able to represent constant functions.

(cherry picked from commit 976bed34)

This class BrezziDouglasMariniSimplexLocalFiniteElement has the domain
dimension and the polynomial order as template parameters, rather
than encoding them into the class name itself.

This class implements Lagrange finite elements on simplices
with compile-time dimension and order. It currently does not
cover more general dimension/order situations than what is
already available in dune-localfunctions, but it gathers the
plethora of different Pk3DNodal, PkNodal, P1Nodal, etc implementations
under one single name.

The implementation is generic, and the patch adds a unit test
that tests all dimensions <= 4.

That way it becomes The One Header To Include(TM) for all
Lagrange Finite elements.

It includes all headers from the `hierarchical` directory.

This is a breaking interface change. Hence we should
document the removal and what to use instead of `clone()`.

If you want a list item withz multiple paragraphs in markdown,
then (at least the first line) of any paragraph following the
first one must be indented by either 4 spaces or a tab. Otherwise
those paragraphs will not be part of the list item, cf.
https://daringfireball.net/projects/markdown/syntax#list

BTW: Don't ask we what to do for second level items with
multiple paragraphs.