Slightly refactor float_factory (for mypy)

[Funth0mas]

So that we can setup mypy on CI we need to elliminate all errors. Currently all serious mypy errors are in canmatrix.canmatrix and most of them are bound to float_factory playing with attrs.

The problem is, that mypy currently only undertsand attr converters, if they are named functions. That means NEITHER converter=decimal.Decimal NOR converter=CanId.from_compound_integer.

Hopefully we could add named 'helper' function (say convert_to_float(x: Any) -> Any)to satisfy mypy, but the question is - do every Signal needs its own converter? Wouldn't be single module-level converter enough?