[CI] Optional expensive tests.

See merge request core/dune-common!523

Spelling in comments.

See merge request core/dune-common!525

Depending on the Simd library, this leads to arrors about list-initialization.

Depending on use, use either the previously introduced `Simd::broadcast<V>()`
function, or in the testsuite to the incantation directly to avoid test
interdependencies.

[Simd] Introduce broadcast<V>(s) function.

See merge request core/dune-common!524

This adds build that runs expensive tests, including Vc on with at least AVX.
The build is allowed to fail to compensate for the possibility of no runner
beeing available.

Feature/float128

See merge request core/dune-common!518

[bugfix] add missing include for `std::integer_sequence`

See merge request core/dune-common!521

[bugfix] VariableSizeCommunicator fixes

See merge request core/dune-common!520

Otherwise we are trying to receive more requests than are there and get
dubious exception because of resizing vectors with MPI_UNDEFINED.

MPI_Wait* and MPI_Test* will ignore these.

Indeed we cannot assume that we always have to send data along one.

[SIMD/test] Do not access indeterminate simd vectors with lane().

See merge request core/dune-common!519

Depending on implementation, this may require bitmask operations on the
indeterminate value, which requires an lvalue-to-rvalue conversion, which
results in undefined behaviour except in some very special cases.

Add helper function unpackIntegerSequence

See merge request core/dune-common!495

[SIMD] Test the index vectors the same way we test other vectors.

Closes #125

See merge request core/dune-common!516

In the tests for operations we check whether an operation is supported on
scalars, and if so, we also test is in simd types.  This suppresses tests for
e.g. `%` on simd types made up of floating point types.  Before this patch we
sometimes did the check on proxies instead of scalars.  This could lead to
tests be incorrectly not suppressed.

This had been watered down because vectorclass does not implement some
functionality for integer vector types (like certain forms of division or
bitshift).

Instead of watering down the test, we're now going to provide that
functionality in a poor efficiency version.

[SIMD] Get rid of IsStandard and all specializations.

Closes #122

See merge request core/dune-common!514

Fixes: #122.

A non-const class rvalue is, in fact, assignable, unless you take special
care.  In the tests we try to apply each operator to `Scalar<V>` in an
unevaluated context, and if that succeeds we require that it works for `V`
analogously to how it does for `Scalar<V>`.

That heuristic fails if `Scalar<V>` is a class (such as `std::complex`),
because assignment works even to rvalues (usually), but trying to apply the
assignment to a `V` rvalue and then capturing and examining the result will
lead to undefined behaviour.

[AlignedNumber] Run the SIMD testsuite.

Closes #123

See merge request core/dune-common!511

[ArithmeticTestSuite] Inherit constructors from TestSuite.

See merge request core/dune-common!512

This makes it possible to e.g. attach a name to the testsuite object.