This is again in line with treatin complex as a vector and lets the
float compare tests pass again.

Complex numbers can take the form `+-1.1+-2.2i`, `+-1.1`, but not
`+-1.1i`. That last one would need to be written as `0+-1.1i`.

There was an unannounced interface change in Parametertree which this
commit works around, while keeping backward compat.

offender was in gtest

also harmonizes from_string arg passing

test creation failed to provide values to compare against. This is now
automated via the VectorAbstraction. VectorAbstraction::create(size=s,
val=y) will create a complex number with real(complex) == y ==
imag(complex). Imo that's consistent with the rest, esp since we decided
to treat complex numbers as vectors anyhow.

Linear function

Add threading.max_count to global config when setting it

refs #59 and also tests float_cmp for actual floats

refs fa6bc47665039871bd256dbba230c1bbae014077

These are actually copy and movable, but not assignable. Note that this
holds for the providers here, not the actually wrapped storage!

The same assert is present in the base above.

This removes the need to include fvector.hh

everything compiles

I just found those and consider them both violating the principle of
least surprise and a source for very hard to find bugs/unwanted
behavior. For example they change the meaning of == for std::vector.
They enable using vector as keys in associative containers, because
std::less might now find an operator
< to call.
It's additionally surprising because the now disabled overloads
only work for std::vectors with default allocator.
None of our own (dune-stuff) tests apparently relied on the changed
comparison, therefore those should be removed as soon as dependent
modules have been examined.

Otherwise `VectorAbstraction` is undefined.

Otherwise std::compelx is undefined.

This will avoid name clashes (at least a bit).