Use Hybrid::equal_to instead of Hybrid::equals

See merge request !486

Add container descriptor for blocked-interleaved index-merging strategy

See merge request !467

Update COPYING

See merge request !484

Add SPDX license information

See merge request !475

[examples][doc] Fix sign of pressure in Stokes example

See merge request !482

This has been changed to the physically correct version in the
example code, too.

While the exact sign is not important mathematically,
the example used the physically wrong sign. Thus this
is also uncommon in literature. The physically correct form
of the Stokes equation is `-\Delta u + \grad p = f` which,
after partial integration leads to a `-div(v)p` term in the
weak form.

This issues exists since the example was added. Maybe the
initial version of the example was implemented based on the
book by Braess. There, indeed the uncommon sign is used for
the Stokes as well as for the Navier-Stokes equation.

No surprisinly, the uncommon convention leads to confusing
results. E.g. for the lid driven cavity flow, the pressure
decreases towards the corner where the fluid is driven to.
Similarly, for Poiseuille flow in a pipe the fluid is driven
towards the high pressure side.

This commit adds the LGPL-3.0-or-later and
GPL-2.0-only-with-DUNE-exception license text files.
These are the one that currently cover most of the code.

The list is taken from the git log.

Remove redundant include of `test-localfe.hh` in gridviewfunctionspacebasistest.cc

See merge request !479

Remove old protocol from the 2014 meeting in Aachen

See merge request !476

use markdown to include figure in readme

See merge request !478

Improve README.md

See merge request !477

In particular, this imports a few bits from the old dune-functions page
in dune-website.  In the future, the website will show the README.md
file directly.

This is just a set of notes, which I think is not of value
to anyone anymore.

[example][python][bugfix] Fix quadrature order in poisson-pq2.py

See merge request !473

When using Q4 ansatz functions, the product of gradients
is still a 4th order polynomial. Hence we should use order 4
for integration. While the solution looks ok with order 3,
it is actually slightly different and one would probably not
get the correct 2nd order convergence.

Bugfix Interpolate Derivatives - Const R-value-Reference replaced by universal reference

See merge request staging/dune-functions!469

Add factory for container descriptors specialized for different pre-basis types

See merge request !454

Add a Python implementation of the mixed Poisson example

See merge request !447

It is not really needed anymore now that we have the new example
'poisson-pq2.py'.

This example program does the same as poisson-mfem.cc, but using
the Python interface.  The only difference is the type of the stiffness
matrix.  The C++ example uses a 2x2 matrix of BCRSMatrix objects,
but that is not easy to replicate in Python unless we start writing
Python bindings for dune-istl.  This new Python example uses
a single flat matrix instead.

Previously it was exported as a property.  But it is a method
in the C++ interface, and therefore for the sake of consistency
it should be a method in Python, too.