Ux/units (#2233)

The core issue here is to add units to the user facing API. I decided on
using the LLNL/units
library, which offers conversion and checking at runtime. Runtime is a
requirement -- as much
as I love static guarantees --, but keeping the interface uniform
between Python and C++ is a
must.

While setting this up, I noticed the severe lack of IDE/LSP support for
Arbor, so I added typing
stubs using https://github.com/sizmailov/pybind11-stubgen. The
conjunction of typing and units
exposed misuse of pybind11 in several places, so next I had to massage
the ordering of bindings,
adjust the specification of default arguments, and add the odd missing
binding.

The schedule/event generator interface was tightened up, hiding the
`*_impl` structs and exposing
only the type erased `schedule` object. That in turn required
de-generification of the Poisson
schedule. Now, Mersenne twister is the only choice and I will remove
that later on for the CBRNG
we are already using elsewhere....