[SIMD] Get rid of IsStandard and all specializations.

Closes #122

See merge request !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 !511

[ArithmeticTestSuite] Inherit constructors from TestSuite.

See merge request !512

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

Also run the simd testsuite on AlignedNumber, sind it does provide a SIMD
abstraction.

Also fixes one issue with that abstraction (though I do not recall why exactly
I did not allow `const Scalar<T> &lane(T &&)`, but required the return type to
be one of `Scalar<T>` or `Scalar<T>&&`).

[Simd::IsStandard] Disable for AlignedNumber too.

See merge request !510

This fixes the tests for istl again.

See !509, #122.

[Simd::IsStandard] Disarm.

Closes #121

See merge request !509

Fixes: #121.

Stop-gap measure to to accept any type in the adaption for standard types.

Minor moderization of pool allocator

See merge request !507

Also use a `std::vector` instead of plain `new[]` and `delete[]`.

[CI] Only run quick tests by default.

See merge request !502

[SIMD] let LoopSIMD use the mathclassifier interface

See merge request !498

[doc] add docstring to `e` and `pi` [ci skip]

See merge request !506

They don't appear in the Doxygen documentation otherwise.

Simplify mathematical constants

See merge request !505

removed the specialization of MathematicalConstants as the generic class MathematicalConstants<Field> already covers these use cases

DuneMPI: add spaces around linker flags

See merge request !503

[Ninja] Fix build system self-destructing on clean

Closes #119

See merge request !501

Same reasoning as 4063acee applies,
however there is no LINK_OPTIONS that could be used instead of
LINK_FLAGS [1].  So just add some spaces to make sure that distinct
options aren't joined by accident.

  [1]: https://gitlab.kitware.com/cmake/cmake/issues/16543

List generated files including `CMAKE_CURRENT_BINARY_DIR` in
`DUNE_INSTANCE_GENERATED`.  This means we do not need to set the source file
property `GENERATED` on the generated files, which is only meant for files
generated during build time (e.g. with `add_custom_command()` or during
generate time (e.g. with `file(GENERATE)`).

Closes: #119

DuneMPI: append flags to `COMPILE_OPTIONS` instead of `COMPILE_FLAGS`

See merge request !499

`COMPILE_FLAGS` is a string, but `set_property(... APPEND ...)`
appends to a list.  If the list of compile flags is not empty this
results in a superfluous semicolon in the compile flags which will
break the compiler invocation.

For example, when `MPI_DUNE_COMPILE_FLAGS=-lpthread`, calling
`add_dune_mpi_flags` twice for the same target would result in the
compiler command containing `-lpthread;-lpthread`.

This patch makes use of the `COMPILE_OPTIONS` property instead which
is a list.

address more gcc 8 warnings

See merge request !497

This addresses the following warning from GCC 8:

    dune/common/test/parameterizedobjectfactorysingleton.cc:21:29:
    warning: unnecessary parentheses in declaration of ‘init’ [-Wparentheses]

The `ArithmeticTestSuite` is also used for `bool`.  This causes
several warnings from GCC 8 about using integer operations on bool
such as

    dune/common/debugalign.hh:175:65: warning:
    ‘~’ on an expression of type bool [-Wbool-operation]

    dune/common/test/arithmetictestsuite.hh:683:22: warning:
    ‘<<’ in boolean context, did you mean ‘<’ ? [-Wint-in-bool-context]

This addresses the following compiler warning from GCC 8:

    warning: catching polymorphic type ‘class Dune::RangeError’
    by value [-Wcatch-value=]

[cmake] Add a DUNE_PYTHON_FORCE_PYTHON_VERSION variable

Closes #118

See merge request !485

Extended SIMD interface

Closes #84

See merge request !193