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.

Currently, units are used for:
- [X] simulation
- [X] schedule/generator
- [x] paintables
- [X] placeables
  - [X] iclamp
  - [X] threshold
- [X] connections
- [X] gap junctions

Adding units to mechanism interfaces is _interesting_ but requires more
work and thought, so
I'll defer that to a later point in time. We'd need to adjust modcc to
expose and **check** units
and devise a scheme to handle missing units.

Generic TODOs; some might spin off into separate issues.
- [x] ~~rename py::iclamp OR cpp::i_clamp for consistency~~ covered by
#2239
- [x] use scale/base for iexpr paintables for consistency with
scaled_mech
- [x] ~~Use CBRNG for Poisson schedule~~ covered by #2243 
- [ ] Automate stub generation. A wishlist item, requires installing
extra software.
- [x] Properly integrate units w/ spack. NB. Units doesn't have a
spackage.

Closes #1983 
Closes #2032

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Co-authored-by: boeschf <48126478+boeschf@users.noreply.github.com>